As a popular and widely-used programming language, JavaScript is an essential skill for any software developer, web developer, or front-end engineer. Whether you are a seasoned JavaScript developer or a novice, preparing for a JavaScript interview can be challenging.
In order to help you prepare for your upcoming JavaScript interview, we have compiled a list of the top JavaScript interview questions that are commonly asked by recruiters and hiring managers. These questions cover a wide range of topics, including the basics of JavaScript, advanced concepts, and common coding problems. By reviewing these questions and practicing your answers, you can feel confident and prepared for your next interview, and increase your chances of landing your dream job. So, let’s dive into the top 20 JavaScript interview questions!
What is JavaScript and what are its key features?
JavaScript is a high-level, interpreted programming language that is used to create interactive web pages and web applications.
Some key features of JavaScript include that it is Client-side scripting, Cross-platform, Object-oriented, Interactivity, Asynchronous programming, Open source and it has Large community.
Overall, JavaScript is a versatile and powerful language that is essential for creating modern, interactive web applications. Its dynamic nature, client-side scripting capabilities, and large community make it an ideal choice for web developers looking to build engaging and responsive web experiences.
What is the difference between “null” and “undefined” in JavaScript?
undefined is a primitive value that is automatically assigned to a variable that has been declared but has not been initialized or has not been assigned a value.
Example:
let x;
console.log(x); // logs “undefined”
null is a value that represents the intentional absence of any object value. It is often used to indicate that a variable has no value or that a function returns no value.
Example:
let y = null;
console.log(y); // logs “null”
In summary, undefined is used when a variable has not been assigned a value, while null is used when a variable intentionally has no value.
What is “hoisting” in JavaScript?
Hoisting is a JavaScript mechanism where variable and function declarations are moved to the top of their scope before code execution. This means that it is possible to use a variable or function before it has been declared in the code.
Example:
x = 5;
console.log(x);
var x;
Even though the variable x is assigned a value before it is declared, the code above will still work and log 5 to the console. This is because the declaration of x is “hoisted” to the top of its scope, which in this case is the global scope.
What is a closure in JavaScript? How does it work?
A closure is a function that has access to variables in its outer scope, even after the outer function has returned. A closure is created when a function is defined inside another function and the inner function captures variables from the outer function’s local scope.
Example:
function outerFunction() {
let outerVariable = “Hello”;
function innerFunction() {
console.log(outerVariable);
}
return innerFunction;
}
let myClosure = outerFunction();
myClosure(); // logs “Hello”
In this example, outerFunction defines a local variable outerVariable and then returns innerFunction. When outerFunction is called and myClosure is assigned to the returned function, innerFunction is created as a closure, which means it captures outerVariable. When myClosure is called, it logs the value of outerVariable even though outerFunction has already returned.
Closures are useful in many situations, such as when you want to create a private variable or function that can only be accessed by certain functions. They can also be used to create functions that remember their state between calls.
What is the “this” keyword in JavaScript? How is it used?
The “this” keyword refers to the object that is currently executing the code. The value of “this” is determined at runtime, based on how a function is invoked.
The value of “this” can be different depending on the context in which it is used.
Example:
In a method of an object, “this” refers to the object itself:
const myObject = {
name: “John”,
sayHello() {
console.log(“Hello, my name is ” + this.name);
}
};
myObject.sayHello(); // logs “Hello, my name is John”
What is event bubbling in JavaScript?
Event bubbling is a mechanism in JavaScript where when an event is triggered on an element, the event is first handled by the element itself, then by its parent element, and so on up the DOM tree until the root element is reached. This process is known as event bubbling because the event “bubbles up” from the target element to its parent elements.
What is the difference between “==” and “===” in JavaScript?
In JavaScript, “==” and “===” are both used to compare two values, but they have different behaviors.
The “==” operator is called the “loose equality” operator. It compares two values for equality after converting both values to a common type. If the values are of different types, JavaScript will attempt to convert one or both values to a common type before comparing.
Example:
console.log(5 == “5”); // logs true
console.log(true == 1); // logs true
console.log(null == undefined); // logs true
In each of these examples, the “==” operator compares two values of different types and returns true because JavaScript converts one or both of the values to a common type before comparing.
The “===” operator is called the “strict equality” operator. It compares two values for equality without converting their types. If the values are of different types, the comparison will always return false. For example:
Example:
console.log(5 === “5”); // logs false
console.log(true === 1); // logs false
console.log(null === undefined); // logs false
In each of these examples, the “===” operator compares two values of different types and returns false because the types are not equal.
What is the difference between “let”, “var”, and “const” in JavaScript?
The “let”, “var”, and “const” are all used to declare variables, but they have different scoping and mutability behaviors.
var is the oldest way to declare variables in JavaScript. It has function-level scope, meaning that variables declared with “var” are accessible throughout the function in which they are declared, regardless of block scope. “var” variables are also mutable, meaning that they can be re-assigned to a new value.
Example:
function example() {
var x = 10;
if (true) {
var x = 20;
console.log(x); // logs 20
}
console.log(x); // logs 20
}
let has block-level scope, meaning that variables declared with “let” are accessible only within the block in which they are declared. “let” variables are also mutable.
Example
function example() {
let x = 10;
if (true) {
let x = 20;
console.log(x); // logs 20
}
console.log(x); // logs 10
}
const has block-level scope, like “let”. However, “const” variables are read-only and cannot be re-assigned to a new value.
Example
function example() {
const x = 10;
if (true) {
const x = 20;
console.log(x); // logs 20
}
console.log(x); // logs 10
}
What is a callback function in JavaScript?
A callback function is a function that is passed as an argument to another function and is executed inside that function. The purpose of a callback function is to allow asynchronous processing or handling of events in JavaScript.
Functions are first-class objects, which means that they can be passed as arguments to other functions just like any other value. When a function is passed as a callback to another function, it is typically executed at a later time, when some event occurs or when a certain condition is met.
Example:
function doSomething(callback) {
console.log(“Doing something…”);
callback();
}
function afterDoSomething() {
console.log(“Done doing something.”);
}
doSomething(afterDoSomething);
In this example, the “doSomething” function takes a callback function as an argument and executes it at the end of its own execution. The “afterDoSomething” function is passed as a callback to “doSomething” and is executed after “doSomething” logs “Doing something…”.
What is the difference between “map” and “forEach” in JavaScript?
Map and forEach are both methods that can be used to iterate over arrays, but they have some differences in their behavior.
forEach is a method that calls a provided function once for each element in an array, in order. The function is called with the element as its argument. The return value of the function is not used. For example:
Example:
let arr = [1, 2, 3, 4, 5];
arr.forEach(function(element) {
console.log(element);
});
This code will log the numbers 1 through 5 to the console, one at a time.
map is a method that creates a new array with the results of calling a provided function on every element in the calling array. The function is called with the element as its argument, and the value returned by the function is added to the new array.
Example:
let arr = [1, 2, 3, 4, 5];
let doubledArr = arr.map(function(element) {
return element * 2;
});
console.log(doubledArr);
This code will create a new array called “doubledArr” that contains the values [2, 4, 6, 8, 10].