Javascript interview questions set 2

For set 1 please refer to the Javascript interview question set 1.

Let's begin with set 2,

3)What is the difference between Object.seal() and Object.freeze() in JavaScript?

Object.seal() method protects the deletion of an existing property

 but it can't protect the existing properties from outside changes.

 var object1 = {

      prop1: 1

  };

  Object.seal(object1);

  object1.prop1 = 2; // value got changed

  delete object1.prop1;

  console.log(object1)

  //It will show value of prop1=2. 

  We can't delete the property. It can just be modified.

In addition to the functionalities of Object.seal(),

The Object.freeze() method even won't allow minute changes to the existing properties of an object.

 var object1 = {

      prop1: 1

  };

 Object.freeze(object1);

 object1.prop1 = 2; 

 delete object1.prop1;

 console.log(object1)

 It will show the value of prop1=1. 

 We can't delete any property or modify any property.

4)What is negative infinity?

The negative infinity in JavaScript is a constant value which is used to represent a value which is the lowest available. 

This means that no other number is lesser than this value and also this might be the use case for using this.

NEGATIVE_INFINITY is a static property of the JavaScript Number object.

Syntax:

Number.NEGATIVE_INFINITY

5)What is hoisting?

Hoisting is JavaScript's default behavior of moving declarations to the top.

The JavaScript engine hoists the variables declared using the let keyword,

 but it doesn’t initialize them as the variables declared with the var keyword.

var variables are initialized with undefined, while let and const are not initialized only they are declared.

Function expressions and arrow functions aren’t hoisted.

Let's check the function hoisting in detail.

Like variables, the JavaScript engine also hoists the function declarations. 

It moves the function declarations to the top of the script. 

For example:

let x = 20,

    y = 10;

let result = add(x,y);

console.log(result);

function add(a, b){

return a + b;

}

In this example, we called the add() function before defining it. The above code is equivalent to the following:

function add(a, b){

    return a + b;

}

let x = 20,

    y = 10;

let result = add(x,y);

console.log(result);

Function expression:

The following example changes the add from a regular function to a function expression:

let x = 20,

    y = 10;

let result = add(x,y);

console.log(result);

var add = function(x, y) {

return x + y;

}

If you execute the code, the following error will occur:

"TypeError: add is not a function   

During the creation phase of the global execution context, the JavaScript Engine creates the add variable in the memory and initializes its value to undefined. When executing the following code, the add is undefined, hence, it isn’t a function:

let result = add(x,y);

The add variable is assigned to an anonymous function only during the execution phase of the global execution context.

I hope you like this article. Please stay connected for set 3.

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Written By:

Saurabh Joshi