JavaScript Quiz Questions and Answers

What is a closure in JavaScript and how do you create one?
What is the difference between == and === in JavaScript?
How do you declare a variable in JavaScript?
What is the difference between null and undefined in JavaScript?
How do you check if a variable is an array in JavaScript?
What is the difference between var and let in JavaScript?
How do you create a new object in JavaScript?
How do you add an element to the end of an array in JavaScript?
How do you loop through an object in JavaScript?
How do you check if a variable is defined in JavaScript?

What is a closure in JavaScript and how do you create one?

A closure is a function that has access to variables in its outer (enclosing) function’s scope chain. This allows the closure to retain access to those variables even after the outer function has returned. Closures are created by defining a function inside another function and returning it. Here is an example:

function outerFunction() {

  let outerVariable = “Hello”;

  function innerFunction() {

    console.log(outerVariable);

  }

  return innerFunction;

}

let closure = outerFunction(); // closure now refers to innerFunction

closure(); // logs “Hello” because innerFunction has access to outerVariable

What is the difference between == and === in JavaScript?

The == operator compares values for equality after performing type coercion, while the === operator compares values for equality without performing type coercion. For example:

1 == “1” // true because “1” is coerced to a number

1 === “1” // false because “1” is not equal to 1 without coercion

How do you declare a variable in JavaScript?

You can declare a variable in JavaScript using the var, let, or const keywords. var and let are used for declaring variables that can be reassigned, while const is used for declaring variables that cannot be reassigned. For example:

var x = 5; // var keyword

let y = 10; // let keyword

const z = 15; // const keyword

What is the difference between null and undefined in JavaScript?

null is a value that represents the intentional absence of any object value, while undefined is a value that represents an uninitialized or nonexistent value. For example:

let x = null; // x is intentionally set to null

let y; // y is undefined because it has not been initialized

How do you check if a variable is an array in JavaScript?

You can check if a variable is an array in JavaScript using the Array.isArray() method. For example:

let x = [1, 2, 3];

let y = “hello”;

console.log(Array.isArray(x)); // true

console.log(Array.isArray(y)); // false

What is the difference between var and let in JavaScript?

The main difference between var and let in JavaScript is that var has function scope, while let has block scope. This means that variables declared with var are accessible within the entire function in which they are declared, while variables declared with let are only accessible within the block in which they are declared. For example:

function example() {

  var x = 5;

  if (true) {

    var x = 10;

  }

  console.log(x); // logs 10 because x is reassigned within the same function scope

}

function example2() {

  let y = 5;

  if (true) {

    let y = 10;

  }

  console.log(y); // logs 5 because y is only accessible within the block scope of the if statement

}

How do you create a new object in JavaScript?

You can create a new object in JavaScript using either object literal notation {}, constructor notation new Object(), or class notation class MyClass {}. For example:

let obj1 = {}; // object literal notation

let obj2 = new Object(); // constructor notation

class MyClass { // class notation

  constructor(x, y){

this.x = x;

this.y = y;

}}

let obj3 = new MyClass(5, 10);

How do you add an element to the end of an array in JavaScript?

You can add an element to the end of an array in JavaScript using the `push()` method. For example:

let arr = [1, 2, 3];

arr.push(4);

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

How do you loop through an object in JavaScript?

You can loop through an object in JavaScript using a `for…in` loop. For example:

let obj = {x: 1, y: 2, z: 3};

for (let prop in obj) {

console.log(${prop}: ${obj[prop]});

}

// logs “x: 1”, “y: 2”, “z: 3”

How do you check if a variable is defined in JavaScript?

You can check if a variable is defined in JavaScript using the `typeof` operator or the `undefined` keyword. For example:

let x;

console.log(typeof x === “undefined”); // true because x has not been initialized

console.log(x === undefined); // true because x has not been initialized