JavaScript Interview Questions

1. Data Types in JavaScript

JavaScript has the following primitive data types:

String: Represents textual data.
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let name = "John";

Number: Represents numeric values, both integer and floating-point.
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let age = 30;

Boolean: Represents true or false.
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let isAdult = true;

Undefined: A variable that has been declared but not assigned a value.
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let x;

console.log(x); // undefined

Null: Represents the intentional absence of any object value.
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let y = null;

Symbol: A unique and immutable primitive value.
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const sym = Symbol("description");

BigInt: For integers of arbitrary length.
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const bigInt = 1234567890123456789012345678901234567890n;

2. Hoisting in JavaScript

Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope during the compile phase. For example:

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console.log(a); // undefined

var a = 5;

foo(); // "Hello"

function foo() {

  console.log("Hello");

}

In the above example, the variable a is hoisted, but its assignment is not, leading to undefined being logged.

3. Debugger in JavaScript

The debugger statement stops the execution of the code and calls the debugger. If no debugging is available, it behaves like a no-op.

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function myFunction() {

  debugger; // Execution will pause here

  console.log("Debugging!");

}

4. Difference between == and ===

• == checks for value equality with type coercion.

• === checks for both value and type equality without coercion.

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console.log(5 == '5'); // true

console.log(5 === '5'); // false

5. Difference between var and let

• var: Function-scoped or globally scoped, can be redeclared.

• let: Block-scoped, cannot be redeclared in the same scope.

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var x = 10;

if (true) {

  var x = 20; // Same variable

}

console.log(x); // 20

let y = 10;

if (true) {

  let y = 20; // Different variable

}

console.log(y); // 10

6. Implicit Type Coercion

JavaScript automatically converts types during operations.

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console.log(1 + '2'); // "12"

console.log('5' - 2); // 3

7. Statically Typed vs. Dynamically Typed

JavaScript is dynamically typed, meaning variables can hold values of any type without explicit type declaration.

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let value = 42;  // Number

value = "Hello"; // String

8. NaN Property

NaN (Not-a-Number) indicates a value that is not a legal number.

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console.log(NaN === NaN); // false

console.log(isNaN("hello")); // true

9. Passed by Value and Passed by Reference

• Passed by Value: Primitive types are copied to the function.

• Passed by Reference: Objects are referenced in memory.

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function updateValue(x) {

  x = 10; // Does not affect the original

}

let a = 5;

updateValue(a);

console.log(a); // 5

function updateObject(obj) {

  obj.key = "new value"; // Affects the original

}

let obj = { key: "value" };

updateObject(obj);

console.log(obj.key); // "new value"

10. Immediately Invoked Function

An Immediately Invoked Function Expression (IIFE) is a function that runs as soon as it is defined.

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(function() {

  console.log("IIFE executed!");

})();

11. Strict Mode

Strict mode is a way to opt into a restricted variant of JavaScript, reducing some silent errors. It can be enabled by adding "use strict"; at the top of a script or function.

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"use strict";

x = 3.14; // ReferenceError: x is not defined

12. Higher Order Functions

Higher order functions are functions that can take other functions as arguments or return functions.

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function greet(name) {

  return function() {

    console.log("Hello, " + name);

  };

}

const greetJohn = greet("John");

greetJohn(); // "Hello, John"

13. this Keyword

this refers to the context in which a function is called. Its value can change depending on how a function is called.

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const obj = {

  name: "Alice",

  greet() {

    console.log("Hello, " + this.name);

  }

};

obj.greet(); // "Hello, Alice"

14. Self Invoking Functions

Self-invoking functions (or IIFEs) are executed immediately after they are defined.

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(function() {

  console.log("Executed!");

})();

15. call(), apply(), and bind()

• call(): Calls a function with a specified this value and arguments.

• apply(): Similar to call(), but takes an array of arguments.

• bind(): Returns a new function, permanently bound to a specific this value.

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function introduce(greeting) {

  console.log(greeting + ", " + this.name);

}

const person = { name: "John" };

introduce.call(person, "Hello"); // "Hello, John"

introduce.apply(person, ["Hi"]); // "Hi, John"

const boundIntroduce = introduce.bind(person);

boundIntroduce("Hey"); // "Hey, John"

16. exec() and test() Methods

• exec(): Executes a search for a match in a string and returns an array or null.

• test(): Tests for a match in a string and returns a boolean.

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const regex = /abc/;

console.log(regex.exec("abcdef")); // ["abc", index: 0, input: "abcdef"]

console.log(regex.test("abcdef")); // true

17. Currying

Currying is a technique of evaluating functions with multiple arguments by transforming them into a sequence of functions, each taking a single argument.

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function multiply(x) {

  return function(y) {

    return x * y;

  };

}

const double = multiply(2);

console.log(double(5)); // 10

18. Advantages of Using External JavaScript

• Improved maintainability by separating code from HTML.

• Code reusability across multiple HTML pages.

• Reduced page load time since files can be cached.

19. Scope and Scope Chain

Scope is the context in which variables are accessible. The scope chain is the hierarchy of scopes that allows variable resolution.

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let outer = "outer";

function outerFunction() {

  let inner = "inner";

  function innerFunction() {

    console.log(outer); // "outer"

    console.log(inner); // "inner"

  }

  innerFunction();

}

outerFunction();

20. Closures

A closure is a function that retains access to its lexical scope, even when the function is executed outside that scope.

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function makeCounter() {

  let count = 0;

  return function() {

    count++;

    return count;

  };

}

const counter = makeCounter();

console.log(counter()); // 1

console.log(counter()); // 2

21. Advantages of JavaScript

• Cross-platform: Runs in any browser.

• Event-driven: Responsive to user actions.

• Asynchronous: Handles operations like network requests without blocking.

• Rich interfaces: Can create dynamic and interactive web pages.

22. Object Prototypes

Every JavaScript object has a prototype, which is another object from which it inherits properties and methods.

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const animal = {

  speak() {

    console.log("Animal speaks");

  }

};

const dog = Object.create(animal);

dog.bark = function() {

  console.log("Woof!");

};

dog.speak(); // "Animal speaks"

dog.bark(); // "Woof!"

23. Callbacks

A callback is a function passed into another function as an argument to be executed later.

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function fetchData(callback) {

  // Simulate async operation

  setTimeout(() => {

    callback("Data fetched");

  }, 1000);

}

fetchData(data => console.log(data)); // "Data fetched" after 1 second

24. Types of Errors in JavaScript

• Syntax Errors: Mistakes in the code that prevent it from running.

• Reference Errors: Accessing a variable that doesn't exist.

• Type Errors: Performing operations on incompatible types.

• Range Errors: Numeric values that are outside the range.

25. Memoization

Memoization is an optimization technique that stores the results of expensive function calls and returns the cached result when the same inputs occur again.

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function memoizedAdd() {

  const cache = {};

  return function(num) {

    if (num in cache) {

      return cache[num];

    }

    const result = num + 10; // Expensive operation

    cache[num] = result;

    return result;

  };

}

const add = memoizedAdd();

console.log(add(5)); // 15

console.log(add(5)); // 15 (cached)

26. Event Delegation

Event delegation is a technique where a single event listener is added to a parent element instead of multiple listeners on child elements. This improves performance and allows handling events for dynamically added elements.

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<ul id="list">

  <li>Item 1</li>

  <li>Item 2</li>

  <li>Item 3</li>

</ul>

<script>

  const list = document.getElementById('list');

  list.addEventListener('click', function(event) {

    if (event.target.tagName === 'LI') {

      console.log(event.target.textContent); // Logs the text of the clicked item

    }

  });

</script>

27. Event Bubbling and Capturing

Event bubbling is the process where events propagate from the target element up to the root. Capturing is the reverse, where events go from the root down to the target.

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<div id="parent">

  <button id="child">Click Me</button>

</div>

<script>

  document.getElementById('parent').addEventListener('click', () => {

    console.log('Parent clicked!');

  }, true); // Capture phase

  document.getElementById('child').addEventListener('click', () => {

    console.log('Child clicked!');

  });

</script>

When the button is clicked, "Child clicked!" will log first, followed by "Parent clicked!" if you capture in the parent.

28. Promises

A promise is an object representing the eventual completion or failure of an asynchronous operation.

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const fetchData = new Promise((resolve, reject) => {

  const success = true; // Simulate success or failure

  if (success) {

    resolve("Data fetched successfully");

  } else {

    reject("Error fetching data");

  }

});

fetchData

  .then(response => console.log(response)) // "Data fetched successfully"

  .catch(error => console.error(error));

29. Async/Await

async functions always return a promise, and await is used to wait for a promise to resolve.

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async function fetchData() {

  try {

    const response = await new Promise((resolve) => {

      setTimeout(() => resolve("Data fetched"), 1000);

    });

    console.log(response); // "Data fetched"

  } catch (error) {

    console.error(error);

  }

}

fetchData();

30. Throttling and Debouncing

• Throttling: Ensures a function is only called at most once in a specified time frame.

• Debouncing: Ensures a function is only called after a specified delay from the last invocation.

Throttling Example:

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function throttle(fn, limit) {

  let lastFunc;

  let lastRan;

  return function() {

    const context = this;

    const args = arguments;

    if (!lastRan) {

      fn.apply(context, args);

      lastRan = Date.now();

    } else {

      clearTimeout(lastFunc);

      lastFunc = setTimeout(() => {

        if ((Date.now() - lastRan) >= limit) {

          fn.apply(context, args);

          lastRan = Date.now();

        }

      }, limit - (Date.now() - lastRan));

    }

  };

}

const throttledLog = throttle(() => console.log('Throttled!'), 2000);

window.addEventListener('resize', throttledLog);

Debouncing Example:

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function debounce(fn, delay) {

  let timeout;

  return function() {

    const context = this;

    const args = arguments;

    clearTimeout(timeout);

    timeout = setTimeout(() => fn.apply(context, args), delay);

  };

}

const debouncedLog = debounce(() => console.log('Debounced!'), 2000);

window.addEventListener('resize', debouncedLog);

31. setTimeout() and setInterval()

• setTimeout(): Executes a function after a specified delay.

• setInterval(): Repeatedly calls a function with a fixed time delay between each call.

Example of setTimeout():

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setTimeout(() => {

  console.log("Executed after 2 seconds");

}, 2000);

Example of setInterval():

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const intervalId = setInterval(() => {

  console.log("Executed every 1 second");

}, 1000);

// To stop the interval after 5 seconds

setTimeout(() => clearInterval(intervalId), 5000);

32. Understanding the Event Loop

The event loop is a mechanism that allows JavaScript to perform non-blocking I/O operations, despite being single-threaded. It executes code, collects and processes events, and executes queued sub-tasks.

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console.log("Start");

setTimeout(() => {

  console.log("Timeout");

}, 0);

Promise.resolve().then(() => console.log("Promise resolved"));

console.log("End");

// Output will be:

// Start

// End

// Promise resolved

// Timeout

33. this in Different Contexts

The value of this depends on how a function is called:

• Global context: this refers to the global object (in browsers, it's window).

• Object method: this refers to the object the method belongs to.

• Constructor function: this refers to the newly created object.

• Event handler: this refers to the element that fired the event.

• Arrow functions: this retains the value of the enclosing lexical context.

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const obj = {

  name: "Alice",

  greet: function() {

    console.log(this.name); // "Alice"

  }

};

obj.greet();

const greet = obj.greet;

greet(); // undefined (or "Alice" in strict mode)

const arrowGreet = () => {

  console.log(this.name); // Refers to lexical scope

};

arrowGreet();

34. Object Destructuring

Object destructuring allows unpacking values from objects into distinct variables.

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const person = {

  name: "John",

  age: 30,

};

const { name, age } = person;

console.log(name); // "John"

console.log(age); // 30

35. Array Destructuring

Array destructuring allows unpacking values from arrays into distinct variables.

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const colors = ["red", "green", "blue"];

const [red, green] = colors;

console.log(red); // "red"

console.log(green); // "green"

36. Spread Operator

The spread operator (...) allows iterables (like arrays) to be expanded into more elements.

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const arr1 = [1, 2, 3];

const arr2 = [4, 5, 6];

const combined = [...arr1, ...arr2];

console.log(combined); // [1, 2, 3, 4, 5, 6]

const obj1 = { a: 1 };

const obj2 = { b: 2 };

const combinedObj = { ...obj1, ...obj2 };

console.log(combinedObj); // { a: 1, b: 2 }

37. Rest Operator

The rest operator (...) collects all remaining elements into an array.

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function sum(...args) {

  return args.reduce((total, num) => total + num, 0);

}

console.log(sum(1, 2, 3)); // 6

38. Array Methods

Common array methods include:

• map(): Creates a new array with the results of calling a function on every element.

• filter(): Creates a new array with all elements that pass the test implemented by the provided function.

• reduce(): Executes a reducer function on each element of the array, resulting in a single output value.

Examples:

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const numbers = [1, 2, 3, 4, 5];

const doubled = numbers.map(num => num * 2);

console.log(doubled); // [2, 4, 6, 8, 10]

const evenNumbers = numbers.filter(num => num % 2 === 0);

console.log(evenNumbers); // [2, 4]

const sum = numbers.reduce((acc, num) => acc + num, 0);

console.log(sum); // 15

39. Object Methods

Methods are functions that belong to an object. They can access the object's properties using this.

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const calculator = {

  add: function(x, y) {

    return x + y;

  },

  multiply: function(x, y) {

    return x * y;

  },

};

console.log(calculator.add(2, 3)); // 5

console.log(calculator.multiply(2, 3)); // 6

40. AJAX and Fetch API

AJAX (Asynchronous JavaScript and XML) allows the browser to send and receive data asynchronously without refreshing the page. The Fetch API provides a modern way to make HTTP requests.

Example using Fetch API:

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fetch('https://api.example.com/data')

  .then(response => {

    if (!response.ok) {

      throw new Error('Network response was not ok');

    }

    return response.json();

  })

  .then(data => console.log(data))

  .catch(error => console.error('There was a problem with the fetch operation:', error));

41. Closures

A closure is a function that retains access to its lexical scope, even when the function is executed outside that scope.

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function outer() {

  const outerVar = 'I am outside!';

  return function inner() {

    console.log(outerVar); // Accesses outerVar

  };

}

const innerFunc = outer();

innerFunc(); // "I am outside!"

42. Modules

JavaScript modules allow for encapsulating code into reusable components. ES6 introduced import and export syntax.

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// module.js

export const PI = 3.14;

export function add(a, b) {

  return a + b;

}

// main.js

import { PI, add } from './module.js';

console.log(PI); // 3.14

console.log(add(2, 3)); // 5

43. Prototypal Inheritance

JavaScript uses prototypal inheritance, where objects can inherit properties and methods from other objects.

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const animal = {

  speak: function() {

    console.log('Animal speaks');

  },

};

const dog = Object.create(animal);

dog.bark = function() {

  console.log('Dog barks');

};

dog.speak(); // "Animal speaks"

dog.bark(); // "Dog barks"

44. map(), filter(), and reduce()

• map(): Transforms each element.

• filter(): Filters elements based on a condition.

• reduce(): Aggregates elements to a single value.

Example:

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const nums = [1, 2, 3, 4, 5];

// Double the numbers

const doubled = nums.map(num => num * 2); // [2, 4, 6, 8, 10]

// Filter even numbers

const evens = nums.filter(num => num % 2 === 0); // [2, 4]

// Sum the numbers

const total = nums.reduce((acc, num) => acc + num, 0); // 15

45. bind(), call(), and apply()

These methods control the value of this in a function.

• call(): Calls a function with a given this value and arguments provided individually.

• apply(): Similar to call(), but takes arguments as an array.

• bind(): Returns a new function with a bound this.

Example:

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const person = {

  name: 'Alice',

  greet: function(greeting) {

    console.log(`${greeting}, ${this.name}`);

  }

};

person.greet('Hello'); // "Hello, Alice"

const greetAlice = person.greet.bind(person);

greetAlice('Hi'); // "Hi, Alice"

const greetUsingCall = person.greet.call(person, 'Hey'); // "Hey, Alice"

const greetUsingApply = person.greet.apply(person, ['Greetings']); // "Greetings, Alice"

46. The new Keyword

The new keyword creates an instance of an object from a constructor function.

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function Car(make, model) {

  this.make = make;

  this.model = model;

}

const myCar = new Car('Toyota', 'Camry');

console.log(myCar.make); // "Toyota"

47. JSON Handling

JSON (JavaScript Object Notation) is a lightweight data interchange format. You can convert between JSON and JavaScript objects using JSON.stringify() and JSON.parse().

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const obj = { name: 'Alice', age: 25 };

const jsonString = JSON.stringify(obj);

console.log(jsonString); // '{"name":"Alice","age":25}'

const parsedObj = JSON.parse(jsonString);

console.log(parsedObj.name); // "Alice"

48. The window Object

The window object is the global object in a browser environment. It represents the browser window and provides access to the DOM, as well as global functions and variables.

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console.log(window.innerWidth); // Width of the browser window

console.log(window.location.href); // Current URL

49. Understanding Scope

Scope determines the accessibility of variables. JavaScript has function scope and block scope (with let and const).

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function testScope() {

  var functionScoped = 'I am function scoped';

  if (true) {

    let blockScoped = 'I am block scoped';

    console.log(blockScoped); // "I am block scoped"

  }

  console.log(functionScoped); // "I am function scoped"

  // console.log(blockScoped); // ReferenceError

}

testScope();

50. Handling Errors with try...catch

Error handling in JavaScript can be managed using try...catch.

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try {

  throw new Error('An error occurred');

} catch (error) {

  console.log(error.message); // "An error occurred"

}

51. Strict Mode

Strict mode is a way to opt into a restricted variant of JavaScript. It helps catch common coding mistakes and "unsafe" actions such as defining global variables.

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'use strict';

function example() {

  x = 3.14; // ReferenceError: x is not defined

}

example();

52. Callbacks

A callback is a function passed as an argument to another function, which is then executed after some operation.

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function fetchData(callback) {

  setTimeout(() => {

    const data = { id: 1, name: 'Alice' };

    callback(data);

  }, 1000);

}

fetchData((data) => {

  console.log(data); // { id: 1, name: 'Alice' }

});

53. The instanceof Operator

The instanceof operator checks whether an object is an instance of a specific constructor or class.

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class Animal {}

class Dog extends Animal {}

const dog = new Dog();

console.log(dog instanceof Dog); // true

console.log(dog instanceof Animal); // true

console.log(dog instanceof Object); // true

54. Template Literals

Template literals provide an easy way to work with strings and include expressions. They use backticks (`) instead of quotes.

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const name = 'John';

const age = 30;

const greeting = `My name is ${name} and I am ${age} years old.`;

console.log(greeting); // "My name is John and I am 30 years old."

55. Default Parameters

Default parameters allow named parameters to be initialized with default values if no value or undefined is passed.

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function greet(name = 'Guest') {

  console.log(`Hello, ${name}!`);

}

greet(); // "Hello, Guest!"

greet('Alice'); // "Hello, Alice!"

56. The for...of Loop

The for...of loop iterates over iterable objects (like arrays, strings, etc.) and provides the value of each element.

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const arr = [1, 2, 3];

for (const num of arr) {

  console.log(num); // 1, then 2, then 3

}

57. The for...in Loop

The for...in loop iterates over the enumerable properties of an object.

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const obj = { a: 1, b: 2, c: 3 };

for (const key in obj) {

  console.log(`${key}: ${obj[key]}`); // "a: 1", "b: 2", "c: 3"

}

58. Set and Map

• Set: A collection of values where each value must be unique.

• Map: A collection of keyed data items, where keys can be any value (including objects).

Set Example:

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const uniqueValues = new Set([1, 2, 3, 3]);

uniqueValues.add(4);

console.log(uniqueValues); // Set { 1, 2, 3, 4 }

Map Example:

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const map = new Map();

map.set('name', 'Alice');

map.set('age', 25);

console.log(map.get('name')); // "Alice"

console.log(map.has('age')); // true

59. The typeof Operator

The typeof operator returns a string indicating the type of the unevaluated operand.

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console.log(typeof 'Hello'); // "string"

console.log(typeof 123); // "number"

console.log(typeof true); // "boolean"

console.log(typeof {}); // "object"

console.log(typeof null); // "object" (this is a known bug)

console.log(typeof undefined); // "undefined"

console.log(typeof function(){}); // "function"

60. The with Statement

The with statement extends the scope chain for a statement. It is generally discouraged because it can lead to confusing code and performance issues.

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const obj = { a: 1, b: 2 };

with (obj) {

  console.log(a); // 1

  console.log(b); // 2

}

61. The delete Operator

The delete operator removes a property from an object.

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const obj = { name: 'Alice', age: 25 };

delete obj.age;

console.log(obj); // { name: 'Alice' }

62. this in Arrow Functions

Arrow functions do not have their own this. Instead, they inherit this from the parent scope.

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const obj = {

  name: 'Alice',

  greet: function() {

    const inner = () => console.log(`Hello, ${this.name}`);

    inner();

  }

};

obj.greet(); // "Hello, Alice"

63. The console Object

The console object provides access to the browser's debugging console. Common methods include console.log(), console.error(), console.warn(), and console.info().

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console.log('This is a log message');

console.warn('This is a warning');

console.error('This is an error');

64. The bind() Method

The bind() method creates a new function that, when called, has its this keyword set to a provided value.

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const person = {

  name: 'Alice',

  greet: function() {

    console.log(`Hello, ${this.name}`);

  }

};

const greetAlice = person.greet.bind(person);

greetAlice(); // "Hello, Alice"

65. Closure with Private Variables

Closures can be used to create private variables that cannot be accessed from outside a function.

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function createCounter() {

  let count = 0;

  return {

    increment: function() {

      count++;

      return count;

    },

    decrement: function() {

      count--;

      return count;

    },

    getCount: function() {

      return count;

    }

  };

}

const counter = createCounter();

console.log(counter.increment()); // 1

console.log(counter.increment()); // 2

console.log(counter.getCount()); // 2

console.log(counter.decrement()); // 1

66. Object Property Shorthand

Object property shorthand allows you to omit the property name when the key and the variable name are the same.

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const name = 'Alice';

const age = 25;

const person = { name, age };

console.log(person); // { name: 'Alice', age: 25 }

67. Object.assign() Method

The Object.assign() method is used to copy the values of all enumerable properties from one or more source objects to a target object.

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const target = { a: 1 };

const source = { b: 2, c: 3 };

const returnedTarget = Object.assign(target, source);

console.log(target); // { a: 1, b: 2, c: 3 }

console.log(returnedTarget); // { a: 1, b: 2, c: 3 }

68. The arguments Object

The arguments object is an array-like object available within functions that contains the values of the arguments passed to the function.

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function sum() {

  let total = 0;

  for (let i = 0; i < arguments.length; i++) {

    total += arguments[i];

  }

  return total;

}

console.log(sum(1, 2, 3)); // 6

69. The window.onload Event

The window.onload event is triggered when the entire page, including all dependent resources, is fully loaded.

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window.onload = function() {

  console.log('Page fully loaded');

};

70. JavaScript Modules (ES6)

JavaScript modules are files that contain code that is executed in strict mode. They can export variables, functions, or classes that can be imported into other modules.

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// math.js

export const add = (a, b) => a + b;

export const subtract = (a, b) => a - b;

// main.js

import { add, subtract } from './math.js';

console.log(add(5, 3)); // 8

console.log(subtract(5, 3)); // 2

71. setInterval() and clearInterval()

setInterval() is used to execute a function repeatedly at specified intervals, while clearInterval() stops the execution.

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const intervalId = setInterval(() => {

  console.log('This will run every 2 seconds');

}, 2000);

// To stop the interval after 10 seconds

setTimeout(() => clearInterval(intervalId), 10000);

72. setTimeout() and clearTimeout()

setTimeout() executes a function after a specified delay, and clearTimeout() cancels the timeout.

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const timeoutId = setTimeout(() => {

  console.log('This will run after 3 seconds');

}, 3000);

// To cancel the timeout

clearTimeout(timeoutId);

73. localStorage and sessionStorage

localStorage allows you to store data with no expiration date. sessionStorage allows you to store data for one session.

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// localStorage example

localStorage.setItem('key', 'value');

console.log(localStorage.getItem('key')); // "value"

// sessionStorage example

sessionStorage.setItem('sessionKey', 'sessionValue');

console.log(sessionStorage.getItem('sessionKey')); // "sessionValue"

74. The fetch() API

The fetch() API is used to make HTTP requests and handle responses.

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fetch('https://api.example.com/data')

  .then(response => response.json())

  .then(data => console.log(data))

  .catch(error => console.error('Error:', error));

75. Regular Expressions

Regular expressions are patterns used to match character combinations in strings. They can be used for searching, replacing, or validating strings.

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const regex = /hello/i;

console.log(regex.test('Hello World')); // true

console.log('Hello World'.replace(regex, 'Hi')); // "Hi World"

76. Prototypal Inheritance

JavaScript uses prototypal inheritance, which means objects can inherit properties and methods from other objects through their prototype chain.

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function Animal(name) {

  this.name = name;

}

Animal.prototype.speak = function() {

  console.log(`${this.name} makes a noise.`);

};

function Dog(name) {

  Animal.call(this, name); // Call the parent constructor

}

Dog.prototype = Object.create(Animal.prototype);

Dog.prototype.bark = function() {

  console.log(`${this.name} barks.`);

};

const dog = new Dog('Rex');

dog.speak(); // "Rex makes a noise."

dog.bark(); // "Rex barks."

77. The Promise Object

A Promise is an object that represents the eventual completion (or failure) of an asynchronous operation and its resulting value.

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const myPromise = new Promise((resolve, reject) => {

  setTimeout(() => {

    resolve('Promise resolved!');

  }, 2000);

});

myPromise.then(result => console.log(result)); // "Promise resolved!" after 2 seconds

78. async and await

async functions return a promise, and await can be used inside them to wait for a promise to resolve.

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async function fetchData() {

  const response = await fetch('https://api.example.com/data');

  const data = await response.json();

  console.log(data);

}

fetchData(); // Fetches data and logs it

79. The bind() Method

The bind() method creates a new function that, when called, has its this keyword set to a provided value.

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const obj = {

  value: 42,

  getValue: function() {

    return this.value;

  }

};

const getValue = obj.getValue.bind(obj);

console.log(getValue()); // 42

80. Object Destructuring

Object destructuring allows you to extract properties from an object into distinct variables.

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const person = { name: 'Alice', age: 25 };

const { name, age } = person;

console.log(name); // "Alice"

console.log(age); // 25

81. Array Destructuring

Array destructuring allows you to unpack values from arrays into distinct variables.

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const colors = ['red', 'green', 'blue'];

const [firstColor, secondColor] = colors;

console.log(firstColor); // "red"

console.log(secondColor); // "green"

82. Spread Operator

The spread operator (...) allows an iterable (like an array) to be expanded in places where zero or more arguments are expected.

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const arr1 = [1, 2, 3];

const arr2 = [4, 5, 6];

const combined = [...arr1, ...arr2];

console.log(combined); // [1, 2, 3, 4, 5, 6]

83. Rest Parameters

Rest parameters allow you to represent an indefinite number of arguments as an array.

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function sum(...numbers) {

  return numbers.reduce((acc, num) => acc + num, 0);

}

console.log(sum(1, 2, 3, 4)); // 10

84. The Map Object

The Map object holds key-value pairs, where keys can be any datatype, and maintains the insertion order.

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const map = new Map();

map.set('name', 'Alice');

map.set(1, 'one');

map.set(true, 'boolean');

console.log(map.get('name')); // "Alice"

console.log(map.size); // 3

85. The Set Object

The Set object lets you store unique values of any type, whether primitive values or object references.

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const set = new Set([1, 2, 2, 3]);

set.add(4);

console.log(set); // Set { 1, 2, 3, 4 }

86. The typeof Operator

The typeof operator returns a string indicating the type of the unevaluated operand.

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console.log(typeof 'Hello'); // "string"

console.log(typeof 123); // "number"

console.log(typeof true); // "boolean"

console.log(typeof undefined); // "undefined"

console.log(typeof {}); // "object"

console.log(typeof []; // "object" (arrays are objects in JavaScript)

console.log(typeof null); // "object"

87. The instanceof Operator

The instanceof operator tests whether an object is an instance of a specific class or constructor.

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class Person {}

const alice = new Person();

console.log(alice instanceof Person); // true

console.log(alice instanceof Object); // true

88. Event Delegation

Event delegation is a technique for managing events by using a single event listener on a parent element instead of multiple listeners on child elements.

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document.getElementById('parent').addEventListener('click', function(event) {

  if (event.target.matches('.child')) {

    console.log('Child clicked:', event.target.textContent);

  }

});

89. Debouncing

Debouncing is a technique used to limit the rate at which a function is executed. It's commonly used with event handlers.

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function debounce(func, delay) {

  let timeout;

  return function(...args) {

    clearTimeout(timeout);

    timeout = setTimeout(() => func.apply(this, args), delay);

  };

}

const log = debounce(() => console.log('Debounced!'), 1000);

window.addEventListener('resize', log);

90. Throttling

Throttling is a technique to ensure a function is only executed at most once in a specified time interval.

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function throttle(func, limit) {

  let lastFunc;

  let lastRan;

  return function() {

    const context = this;

    const args = arguments;

    if (!lastRan) {

      func.apply(context, args);

      lastRan = Date.now();

    } else {

      clearTimeout(lastFunc);

      lastFunc = setTimeout(() => {

        if ((Date.now() - lastRan) >= limit) {

          func.apply(context, args);

          lastRan = Date.now();

        }

      }, limit - (Date.now() - lastRan));

    }

  };

}

const log = throttle(() => console.log('Throttled!'), 2000);

window.addEventListener('scroll', log);

91. Closure

A closure is a function that retains access to its lexical scope, even when the function is executed outside that scope.

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function outerFunction() {

  let count = 0;

  return function innerFunction() {

    count++;

    return count;

  };

}

const counter = outerFunction();

console.log(counter()); // 1

console.log(counter()); // 2

92. Function Currying

Currying is a technique in functional programming where a function takes multiple arguments one at a time.

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function multiply(a) {

  return function(b) {

    return a * b;

  };

}

const double = multiply(2);

console.log(double(5)); // 10

93. The reduce() Method

The reduce() method executes a reducer function on each element of the array, resulting in a single output value.

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const numbers = [1, 2, 3, 4];

const sum = numbers.reduce((acc, curr) => acc + curr, 0);

console.log(sum); // 10

94. The map() Method

The map() method creates a new array populated with the results of calling a provided function on every element in the calling array.

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const numbers = [1, 2, 3];

const squares = numbers.map(num => num * num);

console.log(squares); // [1, 4, 9]

95. The filter() Method

The filter() method creates a new array with all elements that pass the test implemented by the provided function.

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const numbers = [1, 2, 3, 4];

const evenNumbers = numbers.filter(num => num % 2 === 0);

console.log(evenNumbers); // [2, 4]

96. The every() Method

The every() method tests whether all elements in the array pass the test implemented by the provided function.

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const numbers = [2, 4, 6];

const allEven = numbers.every(num => num % 2 === 0);

console.log(allEven); // true

97. The some() Method

The some() method tests whether at least one element in the array passes the test implemented by the provided function.

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const numbers = [1, 2, 3];

const hasEven = numbers.some(num => num % 2 === 0);

console.log(hasEven); // true

98. The find() Method

The find() method returns the value of the first element in the array that satisfies the provided testing function.

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const numbers = [1, 2, 3, 4];

const firstEven = numbers.find(num => num % 2 === 0);

console.log(firstEven); // 2

99. The includes() Method

The includes() method determines whether an array includes a certain value among its entries.

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const fruits = ['apple', 'banana', 'cherry'];

const hasBanana = fruits.includes('banana');

console.log(hasBanana); // true

100. The Array.from() Method

The Array.from() method creates a new, shallow-copied Array instance from an array-like or iterable object.

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const str = 'hello';

const charArray = Array.from(str);

console.log(charArray); // ['h', 'e', 'l', 'l', 'o']

101. The Array.prototype.fill() Method

The fill() method fills all the elements of an array from a start index to an end index with a static value.

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const arr = new Array(5).fill(0);

console.log(arr); // [0, 0, 0, 0, 0]

102. The Array.prototype.concat() Method

The concat() method is used to merge two or more arrays. This method does not change the existing arrays but instead returns a new array.

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const arr1 = [1, 2];

const arr2 = [3, 4];

const merged = arr1.concat(arr2);

console.log(merged); // [1, 2, 3, 4]

103. The Array.prototype.reverse() Method

The reverse() method reverses the elements of an array in place.

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const arr = [1, 2, 3];

arr.reverse();

console.log(arr); // [3, 2, 1]

104. The Array.prototype.sort() Method

The sort() method sorts the elements of an array in place and returns the sorted array.

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const numbers = [4, 1, 3, 2];

numbers.sort();

console.log(numbers); // [1, 2, 3, 4] (lexicographical order)

105. The Array.prototype.splice() Method

The splice() method changes the contents of an array by removing or replacing existing elements and/or adding new elements in place.

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const arr = [1, 2, 3, 4];

arr.splice(1, 2, 5, 6); // Remove 2 elements starting from index 1, and add 5 and 6

console.log(arr); // [1, 5, 6, 4]

106. The Array.prototype.slice() Method

The slice() method returns a shallow copy of a portion of an array into a new array object selected from start to end (not included).

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const arr = [1, 2, 3, 4];

const sliced = arr.slice(1, 3);

console.log(sliced); // [2, 3]

107. The forEach() Method

The forEach() method executes a provided function once for each array element.

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const arr = [1, 2, 3];

arr.forEach(num => console.log(num * 2)); // Logs 2, 4, 6

108. The filter() Method

The filter() method creates a new array with all elements that pass the test implemented by the provided function.

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const numbers = [5, 12, 8, 130, 44];

const filtered = numbers.filter(num => num >= 10);

console.log(filtered); // [12, 130, 44]

109. The map() Method

The map() method creates a new array populated with the results of calling a provided function on every element in the calling array.

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const numbers = [1, 2, 3];

const doubled = numbers.map(num => num * 2);

console.log(doubled); // [2, 4, 6]

110. The findIndex() Method

The findIndex() method returns the index of the first element in the array that satisfies the provided testing function.

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const numbers = [5, 12, 8, 130, 44];

const index = numbers.findIndex(num => num > 10);

console.log(index); // 1 (the index of 12)

111. The Array.prototype.every() Method

The every() method tests whether all elements in the array pass the test implemented by the provided function.

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const numbers = [1, 2, 3, 4];

const allLessThanFive = numbers.every(num => num < 5);

console.log(allLessThanFive); // true

112. The Array.prototype.some() Method

The some() method tests whether at least one element in the array passes the test implemented by the provided function.

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const numbers = [1, 2, 3, 4];

const hasEven = numbers.some(num => num % 2 === 0);

console.log(hasEven); // true

113. The Array.prototype.indexOf() Method

The indexOf() method returns the first index at which a given element can be found in the array, or -1 if it is not present.

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const arr = [2, 5, 9];

const index = arr.indexOf(5);

console.log(index); // 1

114. The Array.prototype.lastIndexOf() Method

The lastIndexOf() method returns the last index at which a given element can be found in the array, or -1 if it is not present.

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const arr = [2, 5, 9, 5];

const index = arr.lastIndexOf(5);

console.log(index); // 3

115. The Array.prototype.fill() Method

The fill() method fills all the elements of an array from a start index to an end index with a static value.

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const arr = [1, 2, 3, 4];

arr.fill(0, 1, 3);

console.log(arr); // [1, 0, 0, 4]

116. The Object.keys() Method

The keys() method returns an array of a given object's own enumerable property names.

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const obj = { name: 'Alice', age: 25 };

const keys = Object.keys(obj);

console.log(keys); // ["name", "age"]

117. The Object.values() Method

The values() method returns an array of a given object's own enumerable property values.

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const obj = { name: 'Alice', age: 25 };

const values = Object.values(obj);

console.log(values); // ["Alice", 25]

118. The Object.entries() Method

The entries() method returns an array of a given object's own enumerable string-keyed property [key, value] pairs.

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const obj = { name: 'Alice', age: 25 };

const entries = Object.entries(obj);

console.log(entries); // [["name", "Alice"], ["age", 25]]

119. The Object.assign() Method

The assign() method is used to copy the values of all enumerable own properties from one or more source objects to a target object.

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const target = { a: 1 };

const source = { b: 2, c: 3 };

const returnedTarget = Object.assign(target, source);

console.log(target); // { a: 1, b: 2, c: 3 }

120. The Object.freeze() Method

The freeze() method freezes an object, preventing new properties from being added to it and marking all existing properties as read-only.

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const obj = { name: 'Alice' };

Object.freeze(obj);

obj.name = 'Bob'; // This will not change the name

console.log(obj.name); // "Alice"

121. The Object.seal() Method

The seal() method seals an object, preventing new properties from being added and marking all existing properties as non-configurable.

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const obj = { name: 'Alice' };

Object.seal(obj);

obj.name = 'Bob'; // This will change the name

delete obj.name; // This will not work

console.log(obj.name); // "Bob"

122. The Object.create() Method

The create() method creates a new object, using an existing object as the prototype of the newly created object.

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const animal = {

  speaks: function() {

    console.log('Animal speaks');

  }

};

const dog = Object.create(animal);

dog.bark = function() {

  console.log('Dog barks');

};

dog.speaks(); // "Animal speaks"

dog.bark(); // "Dog barks"

123. The JSON.stringify() Method

The stringify() method converts a JavaScript object or value to a JSON string.

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const obj = { name: 'Alice', age: 25 };

const jsonString = JSON.stringify(obj);

console.log(jsonString); // '{"name":"Alice","age":25}'

124. The JSON.parse() Method

The parse() method parses a JSON string, constructing the JavaScript value or object described by the string.

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const jsonString = '{"name":"Alice","age":25}';

const obj = JSON.parse(jsonString);

console.log(obj); // { name: 'Alice', age: 25 }

125. The fetch() API

The fetch() API provides a way to make network requests. It returns a promise that resolves to the response.

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fetch('https://api.example.com/data')

  .then(response => response.json())

  .then(data => console.log(data))

  .catch(error => console.error('Error:', error));

126. The Promise.all() Method

The Promise.all() method takes an array of promises and returns a single promise that resolves when all of the promises in the array have resolved.

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const promise1 = Promise.resolve(3);

const promise2 = 42;

const promise3 = new Promise((resolve, reject) => {

  setTimeout(resolve, 100, 'foo');

});

Promise.all([promise1, promise2, promise3]).then(values => {

  console.log(values); // [3, 42, "foo"]

});

127. The async and await Keywords

The async function declaration specifies that the function will return a promise. The await operator is used to wait for a promise to be resolved.

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async function fetchData() {

  const response = await fetch('https://api.example.com/data');

  const data = await response.json();

  console.log(data);

}

fetchData();

128. The this Keyword

The this keyword refers to the object it belongs to. Its value is determined by how a function is called.

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const person = {

  name: 'Alice',

  greet: function() {

    console.log(`Hello, my name is ${this.name}`);

  }

};

person.greet(); // "Hello, my name is Alice"

129. Arrow Functions

Arrow functions provide a more concise syntax for writing function expressions. They also lexically bind the this value.

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const add = (a, b) => a + b;

console.log(add(2, 3)); // 5

const person = {

  name: 'Alice',

  greet: () => {

    console.log(`Hello, my name is ${this.name}`);

  }

};

person.greet(); // "Hello, my name is undefined"

130. The bind() Method

The bind() method creates a new function that, when called, has its this keyword set to the provided value.

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const person = {

  name: 'Alice',

  greet: function() {

    console.log(`Hello, my name is ${this.name}`);

  }

};

const greetAlice = person.greet.bind(person);

greetAlice(); // "Hello, my name is Alice"

131. The call() Method

The call() method calls a function with a given this value and arguments provided individually.

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function greet() {

  console.log(`Hello, my name is ${this.name}`);

}

const person = { name: 'Alice' };

greet.call(person); // "Hello, my name is Alice"

132. The apply() Method

The apply() method calls a function with a given this value and arguments provided as an array.

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function greet(greeting) {

  console.log(`${greeting}, my name is ${this.name}`);

}

const person = { name: 'Alice' };

greet.apply(person, ['Hi']); // "Hi, my name is Alice"

133. The setTimeout() Function

The setTimeout() function sets a timer which executes a function or specified piece of code once the timer expires.

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setTimeout(() => {

  console.log('Executed after 2 seconds');

}, 2000);

134. The setInterval() Function

The setInterval() function repeatedly calls a function or executes code, with a fixed time delay between each call.

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const intervalId = setInterval(() => {

  console.log('Executed every 1 second');

}, 1000);

// To stop the interval after 5 seconds

setTimeout(() => {

  clearInterval(intervalId);

}, 5000);

135. The clearTimeout() Function

The clearTimeout() function cancels a timeout previously established by calling setTimeout().

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const timeoutId = setTimeout(() => {

  console.log('This will not be logged');

}, 2000);

clearTimeout(timeoutId); // Cancels the timeout

136. The clearInterval() Function

The clearInterval() function cancels a timed, repeating action that was established by calling setInterval().

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const intervalId = setInterval(() => {

  console.log('Executed every 1 second');

}, 1000);

clearInterval(intervalId); // Cancels the interval

137. The Math Object

The Math object provides mathematical constants and functions.

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console.log(Math.PI); // 3.141592653589793

console.log(Math.sqrt(16)); // 4

console.log(Math.random()); // Random number between 0 and 1

138. The Math.max() and Math.min() Methods

The Math.max() and Math.min() methods return the largest and smallest of zero or more numbers, respectively.

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console.log(Math.max(1, 2, 3)); // 3

console.log(Math.min(1, 2, 3)); // 1

139. The Math.floor(), Math.ceil(), and Math.round() Methods

• Math.floor() returns the largest integer less than or equal to a given number.

• Math.ceil() returns the smallest integer greater than or equal to a given number.

• Math.round() returns the value of a number rounded to the nearest integer.

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console.log(Math.floor(3.7)); // 3

console.log(Math.ceil(3.2)); // 4

console.log(Math.round(3.5)); // 4

140. The Array.prototype.sort() Method

The sort() method sorts the elements of an array in place and returns the sorted array. The default sort order is according to string Unicode code points.

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const numbers = [4, 2, 5, 1, 3];

numbers.sort((a, b) => a - b); // Ascending order

console.log(numbers); // [1, 2, 3, 4, 5]

141. The Array.prototype.reduce() Method

The reduce() method executes a reducer function (that you provide) on each element of the array, resulting in a single output value.

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const numbers = [1, 2, 3, 4];

const sum = numbers.reduce((accumulator, currentValue) => accumulator + currentValue, 0);

console.log(sum); // 10

142. The Object.assign() Method

The assign() method copies the values of all enumerable own properties from one or more source objects to a target object.

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const target = { a: 1 };

const source = { b: 2, c: 3 };

const returnedTarget = Object.assign(target, source);

console.log(returnedTarget); // { a: 1, b: 2, c: 3 }

143. The String.prototype.split() Method

The split() method splits a string into an array of substrings, and returns the new array.

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const str = 'Hello, world!';

const words = str.split(', ');

console.log(words); // ["Hello", "world!"]

144. The String.prototype.join() Method

The join() method joins all elements of an array into a string.

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const arr = ['Hello', 'world!'];

const str = arr.join(', ');

console.log(str); // "Hello, world!"

145. The String.prototype.replace() Method

The replace() method returns a new string with some or all matches of a pattern replaced by a replacement.

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const str = 'Hello, world!';

const newStr = str.replace('world', 'there');

console.log(newStr); // "Hello, there!"

146. The String.prototype.includes() Method

The includes() method determines whether one string may be found within another string.

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const str = 'Hello, world!';

const hasWorld = str.includes('world');

console.log(hasWorld); // true

147. The String.prototype.indexOf() Method

The indexOf() method returns the index of the first occurrence of a specified value in a string, or -1 if not found.

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const str = 'Hello, world!';

const index = str.indexOf('world');

console.log(index); // 7

148. The String.prototype.slice() Method

The slice() method returns a shallow copy of a portion of a string into a new string object.

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const str = 'Hello, world!';

const sliced = str.slice(7, 12);

console.log(sliced); // "world"

149. The String.prototype.trim() Method

The trim() method removes whitespace from both ends of a string.

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const str = '   Hello, world!   ';

const trimmed = str.trim();

console.log(trimmed); // "Hello, world!"

150. The String.prototype.toUpperCase() and toLowerCase() Methods

The toUpperCase() method converts a string to uppercase, and toLowerCase() converts a string to lowercase.

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const str = 'Hello, World!';

console.log(str.toUpperCase()); // "HELLO, WORLD!"

console.log(str.toLowerCase()); // "hello, world!"

151. The Array.prototype.shift() Method

The shift() method removes the first element from an array and returns that removed element. This method changes the length of the array.

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const arr = [1, 2, 3];

const firstElement = arr.shift();

console.log(firstElement); // 1

console.log(arr); // [2, 3]

152. The Array.prototype.unshift() Method

The unshift() method adds one or more elements to the beginning of an array and returns the new length of the array.

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const arr = [2, 3];

const newLength = arr.unshift(1);

console.log(newLength); // 3

console.log(arr); // [1, 2, 3]

153. The Array.prototype.pop() Method

The pop() method removes the last element from an array and returns that element. This method changes the length of the array.

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const arr = [1, 2, 3];

const lastElement = arr.pop();

console.log(lastElement); // 3

console.log(arr); // [1, 2]

154. The Array.prototype.push() Method

The push() method adds one or more elements to the end of an array and returns the new length of the array.

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const arr = [1, 2];

const newLength = arr.push(3);

console.log(newLength); // 3

console.log(arr); // [1, 2, 3]

155. The Array.prototype.includes() Method

The includes() method determines whether an array includes a certain value among its entries, returning true or false.

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const arr = [1, 2, 3];

const hasTwo = arr.includes(2);

console.log(hasTwo); // true

156. The Array.prototype.reduceRight() Method

The reduceRight() method executes a reducer function on each element of the array, from right to left, reducing it to a single value.

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const arr = [1, 2, 3];

const reversedSum = arr.reduceRight((acc, current) => acc + current, 0);

console.log(reversedSum); // 6 (3 + 2 + 1)

157. The Object.prototype.hasOwnProperty() Method

The hasOwnProperty() method returns a boolean indicating whether the object has the specified property as its own property.

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const obj = { name: 'Alice' };

console.log(obj.hasOwnProperty('name')); // true

console.log(obj.hasOwnProperty('age')); // false

158. The String.prototype.charAt() Method

The charAt() method returns the character at a specified index in a string.

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const str = 'Hello';

const char = str.charAt(1);

console.log(char); // "e"

159. The String.prototype.split() Method

The split() method splits a string into an array of substrings, and returns the new array.

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const str = 'a,b,c';

const arr = str.split(',');

console.log(arr); // ["a", "b", "c"]

160. The String.prototype.lastIndexOf() Method

The lastIndexOf() method returns the index of the last occurrence of a specified value in a string.

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const str = 'Hello, world! Hello!';

const index = str.lastIndexOf('Hello');

console.log(index); // 13

161. The String.prototype.match() Method

The match() method retrieves the matches when matching a string against a regular expression.

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const str = 'The rain in SPAIN stays mainly in the plain';

const matches = str.match(/ain/g);

console.log(matches); // ["ain", "ain", "ain"]

162. The String.prototype.search() Method

The search() method executes a search for a match between a regular expression and this String object.

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const str = 'The quick brown fox';

const index = str.search(/quick/);

console.log(index); // 4

163. The String.prototype.slice() Method

The slice() method extracts a section of a string and returns it as a new string, without modifying the original string.

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const str = 'Hello, world!';

const sliced = str.slice(0, 5);

console.log(sliced); // "Hello"

164. The String.prototype.replaceAll() Method

The replaceAll() method returns a new string with all matches of a pattern replaced by a replacement.

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const str = 'foo bar foo';

const newStr = str.replaceAll('foo', 'baz');

console.log(newStr); // "baz bar baz"

165. The String.prototype.startsWith() Method

The startsWith() method determines whether a string begins with the characters of a specified string.

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const str = 'Hello, world!';

console.log(str.startsWith('Hello')); // true

166. The String.prototype.endsWith() Method

The endsWith() method determines whether a string ends with the characters of a specified string.

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const str = 'Hello, world!';

console.log(str.endsWith('world!')); // true

167. The String.prototype.valueOf() Method

The valueOf() method returns the primitive value of a String object.

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const str = new String('Hello');

console.log(str.valueOf()); // "Hello"

168. The Number.isNaN() Method

The isNaN() method determines whether the passed value is NaN and its type is Number.

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console.log(Number.isNaN(NaN)); // true

console.log(Number.isNaN(5)); // false

169. The Number.isInteger() Method

The isInteger() method determines whether the passed value is an integer.

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console.log(Number.isInteger(4)); // true

console.log(Number.isInteger(4.5)); // false

170. The Number.parseFloat() Method

The parseFloat() function parses a string argument and returns a floating point number.

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const num = Number.parseFloat('3.14');

console.log(num); // 3.14

171. The Number.parseInt() Method

The parseInt() function parses a string argument and returns an integer of the specified radix.

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const num = Number.parseInt('10', 2);

console.log(num); // 2 (binary)

172. The Array.from() Method

The from() method creates a new, shallow-copied Array instance from an array-like or iterable object.

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const str = 'Hello';

const chars = Array.from(str);

console.log(chars); // ['H', 'e', 'l', 'l', 'o']

173. The Array.of() Method

The of() method creates a new Array instance with a variable number of arguments, regardless of number or type of the arguments.

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const arr = Array.of(1, 2, 3);

console.log(arr); // [1, 2, 3]

174. The Set Object

The Set object lets you store unique values of any type, whether primitive values or object references.

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const uniqueNumbers = new Set([1, 2, 2, 3]);

console.log(uniqueNumbers); // Set { 1, 2, 3 }

175. The Map Object

The Map object holds key-value pairs and remembers the original insertion order of the keys.

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const map = new Map();

map.set('name', 'Alice');

map.set('age', 25);

console.log(map.get('name')); // "Alice"

176. The WeakSet Object

The WeakSet is a collection of objects, and its references are weakly held. This means that if there are no other references to the object, it can be garbage collected.

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const obj = {};

const weakSet = new WeakSet();

weakSet.add(obj);

console.log(weakSet.has(obj)); // true

177. The WeakMap Object

The WeakMap is a collection of key-value pairs where keys are objects and values can be any value. The references to the keys are weakly held.

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const weakMap = new WeakMap();

const obj = {};

weakMap.set(obj, 'value');

console.log(weakMap.get(obj)); // "value"

178. The Promise Object

The Promise object represents the eventual completion (or failure) of an asynchronous operation and its resulting value.

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const myPromise = new Promise((resolve, reject) => {

  setTimeout(() => {

    resolve('Success!');

  }, 1000);

});

myPromise.then(result => console.log(result)); // "Success!" after 1 second

179. The Promise.allSettled() Method

The allSettled() method returns a promise that resolves after all of the given promises have either resolved or rejected, with an array of objects that each describe the outcome of each promise.

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const p1 = Promise.resolve(3);

const p2 = Promise.reject('error');

const p3 = Promise.resolve('done');

Promise.allSettled([p1, p2, p3]).then(results => {

  console.log(results);

  // [

  //   { status: 'fulfilled', value: 3 },

  //   { status: 'rejected', reason: 'error' },

  //   { status: 'fulfilled', value: 'done' }

  // ]

});

180. The Promise.race() Method

The race() method returns a promise that resolves or rejects as soon as one of the promises in the iterable resolves or rejects, with the value or reason from that promise.

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const p1 = new Promise((resolve) => setTimeout(resolve, 100, 'one'));

const p2 = new Promise((resolve) => setTimeout(resolve, 200, 'two'));

Promise.race([p1, p2]).then(value => {

  console.log(value); // "one" (first promise to resolve)

});

181. The async and await Keywords

The async keyword is used to declare an asynchronous function, and the await keyword is used to wait for a promise to resolve within an async function.

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const fetchData = async () => {

  const response = await fetch('https://api.example.com/data');

  const data = await response.json();

  return data;

};

fetchData().then(data => console.log(data));

182. The EventEmitter Class

The EventEmitter class is part of the Node.js events module and is used to handle events in Node.js.

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const EventEmitter = require('events');

const myEmitter = new EventEmitter();

myEmitter.on('event', () => {

  console.log('An event occurred!');

});

myEmitter.emit('event'); // "An event occurred!"

183. The fetch API

The fetch API provides a JavaScript interface for accessing and manipulating parts of the HTTP pipeline, such as requests and responses.

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fetch('https://api.example.com/data')

  .then(response => response.json())

  .then(data => console.log(data))

  .catch(error => console.error('Error:', error));

184. The setTimeout() Function

The setTimeout() function executes a function, after a specified number of milliseconds.

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console.log('Start');

setTimeout(() => {

  console.log('Timeout executed!');

}, 1000);

console.log('End');

// Output:

// Start

// End

// Timeout executed! (after 1 second)

185. The setInterval() Function

The setInterval() function calls a function or executes a code snippet repeatedly, with a fixed time delay between each call.

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let count = 0;

const intervalId = setInterval(() => {

  console.log(`Count: ${count}`);

  count++;

  if (count === 5) {

    clearInterval(intervalId); // Clear interval after 5 counts

  }

}, 1000);

186. The clearTimeout() and clearInterval() Functions

clearTimeout() and clearInterval() are used to cancel a timeout or an interval, respectively.

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const timeoutId = setTimeout(() => {

  console.log('This will not run.');

}, 1000);

clearTimeout(timeoutId); // Cancels the timeout

187. The Array.prototype.find() Method

The find() method returns the value of the first element in the array that satisfies the provided testing function.

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const arr = [5, 12, 8, 130, 44];

const found = arr.find(element => element > 10);

console.log(found); // 12

188. The Array.prototype.findIndex() Method

The findIndex() method returns the index of the first element in the array that satisfies the provided testing function.

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const arr = [5, 12, 8, 130, 44];

const index = arr.findIndex(element => element > 10);

console.log(index); // 1

189. The Array.prototype.every() Method

The every() method tests whether all elements in the array pass the test implemented by the provided function.

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const arr = [1, 30, 39, 29, 10, 13];

const allLessThan40 = arr.every(element => element < 40);

console.log(allLessThan40); // true

190. The Array.prototype.some() Method

The some() method tests whether at least one element in the array passes the test implemented by the provided function.

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const arr = [1, 30, 39, 29, 10, 13];

const someLessThan10 = arr.some(element => element < 10);

console.log(someLessThan10); // true

191. The Array.prototype.flat() Method

The flat() method creates a new array with all sub-array elements concatenated into it recursively up to the specified depth.

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const arr = [1, 2, [3, 4, [5, 6]]];

const flattened = arr.flat(2);

console.log(flattened); // [1, 2, 3, 4, 5, 6]

192. The Array.prototype.flatMap() Method

The flatMap() method first maps each element using a mapping function, then flattens the result into a new array.

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const arr = [1, 2, 3];

const doubled = arr.flatMap(x => [x, x * 2]);

console.log(doubled); // [1, 2, 2, 4, 3, 6]

193. The Object.assign() Method

The assign() method is used to copy the values of all enumerable own properties from one or more source objects to a target object.

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const target = { a: 1 };

const source = { b: 2, c: 3 };

const returnedTarget = Object.assign(target, source);

console.log(returnedTarget); // { a: 1, b: 2, c: 3 }

194. The Object.keys() Method

The keys() method returns an array of a given object's own enumerable property names.

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const obj = { a: 1, b: 2, c: 3 };

const keys = Object.keys(obj);

console.log(keys); // ['a', 'b', 'c']

195. The Object.values() Method

The values() method returns an array of a given object's own enumerable property values.

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const obj = { a: 1, b: 2, c: 3 };

const values = Object.values(obj);

console.log(values); // [1, 2, 3]

196. The Object.entries() Method

The entries() method returns an array of a given object's own enumerable string-keyed property [key, value] pairs.

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const obj = { a: 1, b: 2, c: 3 };

const entries = Object.entries(obj);

console.log(entries); // [['a', 1], ['b', 2], ['c', 3]]

197. The JSON.stringify() Method

The stringify() method converts a JavaScript object or value to a JSON string.

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const obj = { name: 'Alice', age: 25 };

const jsonString = JSON.stringify(obj);

console.log(jsonString); // '{"name":"Alice","age":25}'

198. The JSON.parse() Method

The parse() method parses a JSON string, constructing the JavaScript value or object described by the string.

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const jsonString = '{"name":"Alice","age":25}';

const obj = JSON.parse(jsonString);

console.log(obj); // { name: 'Alice', age: 25 }

199. The Function.prototype.bind() Method

The bind() method creates a new function that, when called, has its this keyword set to the provided value.

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const obj = { value: 42 };

function getValue() {

  return this.value;

}

const boundGetValue = getValue.bind(obj);

console.log(boundGetValue()); // 42

200. The Function.prototype.call() and Function.prototype.apply() Methods

The call() method calls a function with a given this value and arguments provided individually, while the apply() method calls a function with a given this value and arguments provided as an array.

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function greet(greeting) {

  return `${greeting}, ${this.name}!`;

}

const user = { name: 'Alice' };

console.log(greet.call(user, 'Hello')); // "Hello, Alice!"

console.log(greet.apply(user, ['Hi'])); // "Hi, Alice!"

201. The Function.prototype.apply() Method

The apply() method calls a function with a given this value and arguments provided as an array.

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function add(a, b) {

  return a + b;

}

const result = add.apply(null, [5, 10]);

console.log(result); // 15

202. The Function.prototype.call() Method

The call() method calls a function with a given this value and arguments provided individually.

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function greet(greeting, punctuation) {

  return `${greeting}, ${this.name}${punctuation}`;

}

const person = { name: 'Bob' };

console.log(greet.call(person, 'Hello', '!')); // "Hello, Bob!"

203. The Math Object

The Math object is a built-in object that has properties and methods for mathematical constants and functions.

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const radius = 5;

const area = Math.PI * Math.pow(radius, 2);

console.log(area); // 78.53981633974483

204. The Date Object

The Date object is used to work with dates and times in JavaScript.

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const date = new Date();

console.log(date); // Current date and time

console.log(date.getFullYear()); // Current year

205. The Array.prototype.sort() Method

The sort() method sorts the elements of an array in place and returns the sorted array.

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const numbers = [5, 3, 8, 1];

numbers.sort((a, b) => a - b);

console.log(numbers); // [1, 3, 5, 8]

206. The Array.prototype.reverse() Method

The reverse() method reverses the elements of an array in place.

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const numbers = [1, 2, 3, 4];

numbers.reverse();

console.log(numbers); // [4, 3, 2, 1]

207. The Array.prototype.concat() Method

The concat() method is used to merge two or more arrays.

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const arr1 = [1, 2];

const arr2 = [3, 4];

const combined = arr1.concat(arr2);

console.log(combined); // [1, 2, 3, 4]

208. The Array.prototype.splice() Method

The splice() method changes the contents of an array by removing or replacing existing elements and/or adding new elements.

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const arr = [1, 2, 3, 4];

arr.splice(1, 2, 5, 6); // Removes 2 elements starting from index 1 and adds 5 and 6

console.log(arr); // [1, 5, 6, 4]

209. The Array.prototype.slice() Method

The slice() method returns a shallow copy of a portion of an array into a new array object.

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const arr = [1, 2, 3, 4];

const sliced = arr.slice(1, 3);

console.log(sliced); // [2, 3]

210. The Array.prototype.reduce() Method

The reduce() method executes a reducer function on each element of the array, resulting in a single output value.

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const arr = [1, 2, 3, 4];

const sum = arr.reduce((accumulator, currentValue) => accumulator + currentValue, 0);

console.log(sum); // 10

211. The Array.prototype.reduceRight() Method

The reduceRight() method executes a reducer function on each element of the array, from right to left.

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const arr = [1, 2, 3, 4];

const product = arr.reduceRight((acc, val) => acc * val, 1);

console.log(product); // 24

212. The Array.prototype.fill() Method

The fill() method fills all the elements of an array from a start index to an end index with a static value.

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const arr = new Array(3).fill(0);

console.log(arr); // [0, 0, 0]

213. The Array.prototype.includes() Method

The includes() method determines whether an array includes a certain value among its entries.

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const arr = [1, 2, 3];

const includesTwo = arr.includes(2);

console.log(includesTwo); // true

214. The Set Object

The Set object lets you store unique values of any type.

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const uniqueNumbers = new Set([1, 2, 2, 3]);

console.log(uniqueNumbers); // Set { 1, 2, 3 }

215. The Map Object

The Map object holds key-value pairs, and keys can be of any type.

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const map = new Map();

map.set('a', 1);

map.set('b', 2);

console.log(map.get('a')); // 1

216. The WeakSet Object

The WeakSet is a collection of objects, where the references to the objects are weakly held.

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const weakSet = new WeakSet();

const obj = {};

weakSet.add(obj);

console.log(weakSet.has(obj)); // true

217. The WeakMap Object

The WeakMap is a collection of key-value pairs, where keys are objects and values can be any type.

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const weakMap = new WeakMap();

const obj = {};

weakMap.set(obj, 'value');

console.log(weakMap.get(obj)); // 'value'

218. The Promise Object

The Promise object represents the eventual completion (or failure) of an asynchronous operation and its resulting value.

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const myPromise = new Promise((resolve, reject) => {

  setTimeout(() => {

    resolve('Success!');

  }, 1000);

});

myPromise.then(result => console.log(result)); // 'Success!' (after 1 second)

219. The Promise.all() Method

The Promise.all() method takes an iterable of Promise objects and, when all of the promises have been resolved, returns a single Promise that resolves to an array of the results.

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const promise1 = Promise.resolve(3);

const promise2 = 42;

const promise3 = new Promise((resolve) => setTimeout(resolve, 100, 'foo'));

Promise.all([promise1, promise2, promise3]).then(values => {

  console.log(values); // [3, 42, 'foo']

});

220. The Promise.race() Method

The Promise.race() method returns a promise that resolves or rejects as soon as one of the promises in the iterable resolves or rejects.

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const promise1 = new Promise((resolve) => setTimeout(resolve, 500, 'one'));

const promise2 = new Promise((resolve) => setTimeout(resolve, 100, 'two'));

Promise.race([promise1, promise2]).then(value => {

  console.log(value); // 'two'

});

221. The async/await Syntax

The async function is a function that always returns a promise, and the await keyword is used to wait for a promise to resolve.

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async function fetchData() {

  return 'Data fetched!';

}

fetchData().then(console.log); // 'Data fetched!'

222. Handling Errors with try/catch

You can use try and catch blocks to handle errors in asynchronous functions.

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async function fetchData() {

  throw new Error('Fetch error!');

}

fetchData().catch(error => {

  console.error(error.message); // 'Fetch error!'

});

223. The fetch() API

The fetch() API is a modern way to make network requests. It returns a promise that resolves to the Response object representing the response to the request.

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fetch('https://api.example.com/data')

  .then(response => response.json())

  .then(data => console.log(data));

224. The window.localStorage API

The localStorage API allows you to store data in the web browser persistently.

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localStorage.setItem('username', 'Alice');

console.log(localStorage.getItem('username')); // 'Alice'

225. The window.sessionStorage API