JavaScript Interview Questions

What are the different data types present in javascript?
To know the type of a JavaScript variable, we can use the typeof operator.

1. Primitive types

String - It represents a series of characters and is written with quotes. A string can be represented using a single or a double quote.

Example :

var str = "Vivek Singh Bisht"; //using double quotes
var str2 = 'John Doe'; //using single

1. Explain Hoisting in javascript. Hoisting is the default behaviour of javascript where all the variable and function declarations are moved on top.

This means that irrespective of where the variables and functions are declared, they are moved on top of the scope. The scope can be both local and global.

Example 1:

hoistedVariable = 3;
console.log(hoistedVariable); // outputs 3 even when the variable is declared after it is initialized

var hoistedVariable;

Why do we use the word “debugger” in javascript?
The debugger for the browser must be activated in order to debug the code. Built-in debuggers may be switched on and off, requiring the user to report faults. The remaining section of the code should stop execution before moving on to the next line while debugging.

1. Difference between “ == “ and “ === “ operators. Both are comparison operators. The difference between both the operators is that “==” is used to compare values whereas, “ === “ is used to compare both values and types.

Example:

var x = 2;
var y = "2";
(x == y) // Returns true since the value of both x and y is the same
(x === y) // Returns false since the typeof x is "number" and typeof y is "string"

1. Difference between var and let keyword in javascript. Some differences are

From the very beginning, the 'var' keyword was used in JavaScript programming whereas the keyword 'let' was just added in 2015.
The keyword 'Var' has a function scope. Anywhere in the function, the variable specified using var is accessible but in ‘let’ the scope of a variable declared with the 'let' keyword is limited to the block in which it is declared. Let's start with a Block Scope.
In ECMAScript 2015, let and const are hoisted but not initialized. Referencing the variable in the block before the variable declaration results in a ReferenceError because the variable is in a "temporal dead zone" from the start of the block until the declaration is processed.

javascript.
Implicit type coercion in javascript is the automatic conversion of value from one data type to another. It takes place when the operands of an expression are of different data types.

String coercion
String coercion takes place while using the ‘ + ‘ operator. When a number is added to a string, the number type is always converted to the string type.

Example 1:

var x = 3;
var y = "3";
x + y // Returns "33"

Is javascript a statically typed or a dynamically typed language?
JavaScript is a dynamically typed language. In a dynamically typed language, the type of a variable is checked during run-time in contrast to a statically typed language, where the type of a variable is checked during compile-time.