Javascript interview questions: Unlock top 10 Q’s with A’s

In this article we will look into top 10 Javascript interview questions and their answers. JavaScript is a scripting language used for creating dynamic and interactive websites. It is an object-oriented language that can be run on both the client-side and server-side of the web.

Javascript interview questions

What is closure in JavaScript?

A closure is a function that has access to its outer function’s variables, even after the outer function has returned. This is possible because the inner function retains a reference to its outer function’s variables. For example

 function outer() {
   var a = 1;
   function inner() {
     console.log(a);
   }
   return inner;
 }
 
 var innerFn = outer();
 innerFn(); // logs 1

What is the difference between undefined and null in JavaScript?

undefined means a variable has been declared but has not been assigned a value, whereas null is an assignment value representing no value or no object. For example,

var a;
console.log(a); // logs undefined

var b = null;
console.log(b); // logs null

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

You can use the Array.isArray() method to check if a variable is an array. For example,

var arr = [1, 2, 3];
console.log(Array.isArray(arr)); // logs true

var obj = { name: "John", age: 25 };
console.log(Array.isArray(obj)); // logs false

What is the difference between let, const, and var in JavaScript?

var declares a variable that is function-scoped, whereas let and const declare variables that are block-scoped. const is used to declare constants that cannot be reassigned, whereas let declares variables that can be reassigned. For example,

function test() {
  var a = 1;
  let b = 2;
  const c = 3;
  if (true) {
    var a = 4; // function-scoped
    let b = 5; // block-scoped
    const c = 6; // block-scoped
    console.log(a, b, c); // logs 4 5 6
  }
  console.log(a, b, c); // logs 4 2 3
}

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

== compares the values of two variables, whereas === compares the values and types of two variables. For example,

console.log(1 == "1"); // logs true
console.log(1 === "1"); // logs false

How do you create an object in JavaScript?

You can create an object using object literal notation, constructor function, or the Object.create() method. For example,

// using object literal notation
var person = { name: "John", age: 25 };

// using constructor function
function Person(name, age) {
  this.name = name;
  this.age = age;
}
var person = new Person("John", 25);

// using Object.create() method
var person = Object.create(null);
person.name = "John";
person.age = 25;

What is the difference between let and var in terms of hoisting?

var variables are hoisted to the top of their function or global scope, whereas let variables are not hoisted. For example,

function test() {
  console.log(a); // logs undefined
  var a = 1;
  console.log(b); // ReferenceError: b is not defined
  let b = 2;
}

What is the difference between null and undefined in JavaScript?

undefined means a variable has been declared but has not been assigned a value, whereas null is an assignment value representing no value or no object. For example,

var a;
console.log(a); // logs undefined

var b = null;
console.log(b); // logs null

What is the difference between Object.keys() and Object.getOwnPropertyNames() in JavaScript?

Object.keys() returns an array of the enumerable property names of an object, whereas Object.getOwnPropertyNames() returns an array of all property names, including non enumerable ones. For example,

 var obj = {
   a: 1,
   b: 2
 };
 
 console.log(Object.keys(obj)); // logs ["a", "b"]
 console.log(Object.getOwnPropertyNames(obj)); // logs ["a", "b"]
 
 Object.defineProperty(obj, "c", {
   value: 3,
   enumerable: false
 });
 
 console.log(Object.keys(obj)); // logs ["a", "b"]
 console.log(Object.getOwnPropertyNames(obj)); // logs ["a", "b", "c"]

What is the difference between slice() and splice() in JavaScript?

slice() returns a shallow copy of a portion of an array, whereas splice() modifies the original array by adding or removing elements. For example,

 var arr = [1, 2, 3, 4, 5];
 
 var newArr = arr.slice(1, 3); // returns [2, 3]
 console.log(newArr); // logs [2, 3]
 
 arr.splice(1, 2, 6, 7); // modifies arr to [1, 6, 7, 4, 5]
 console.log(arr); // logs [1, 6, 7, 4, 5]

Hope this helps you get clarified on some of the interview related question. Keep learning. 🙂

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