JavaScript Interview Questions 1 - Data Types

Published on 17 Nov, 2020

Question 1:

What are the data types we have in latest JavaScript?


We have 8 data types. They are:

  1. Boolean
  2. null
  3. undefined
  4. Number
  5. BigInt
  6. String
  7. Symbol
  8. Object

Question 2:

What are falsy values in JavaScript? List the values that are falsy in JavaScript.


Those values when converted to a Boolean type results in false are called falsy values. Here are the list of falsy values in JavaScript.

  • 0
  • -0
  • null
  • false Obviously
  • NaN
  • undefined
  • "" Empty string

Question 3:

What is the output of following code? Please explain your answer.

if ({}) {
  console.log("I am true");
} else {
  console.log("I am false");


It prints "I am true" in the console. Any object, even empty object is a truthy value in JavaScript.


Question 4:

In JavaScript, how can we convert a value to boolean type?


One way is to use Boolean() function.

console.log(Boolean("A string")); // true
console.log(Boolean("")); // false
console.log(Boolean(0)); // false
console.log(Boolean({})); // true

Another way is to use double negation operator !!.

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

Question 5:

What is the difference between null and undefined?


Both null and undefined behaves almost the same way when used in code. But as part of best practice, difference between those two lies in their usage.

null represents intentional absence of an object. That means, if there is a place where the code expects an object, but we are unable to provide one, then we give back null. Example: match() is a string method that returns an array of all matches of a supplied pattern.

const str = "Orange is orange";
const result = str.match(/orange/gi);
console.log(result); // ["Orange", "orange"]

Here the match() method returned an array. After all, array is a type of object. So, we can say that, result is always expecting an object. If there was no match found, then there is no object to return to result. In order to state an intentional absence of an object, match() method returns null if no match is found.

const str = "Orange is orange";
const result = str.match(/Apple/gi);
console.log(result); // null

On the other hand, undefined is the value of a variable that is declared, but not initialized. Also, a function in JavaScript by default returns undefined. So in those context, we can use undefined.


Question 6:

I need to check if the value of variable a is undefined. For that, I wrote the following code:

if (a === undefined) {
  console.log("a is declared");
} else {
  console.log("a is declared and initialized");

What happened is, if a is not declared, above code throws ReferenceError in the first line. How can we resolve this issue?


We can use typeof operator.

if (typeof a === "undefined") {
  // ...

typeof does not throw error for undeclared variables.