JavaScript new Keyword

JavaScript new keyword is used to create an instance of an object that has a constructor function.

On calling the constructor function with the 'new' operator, the following actions are taken:

• A new empty object is created.
• The new object’s internal 'Prototype' property (__proto__) is set the same as the prototype of the constructing function.
• The 'this' variable is made to point to the newly created object. It binds the property that is declared with the 'this' keyword to the new object.
• About the returned value, there are three situations below. 

Syntax:

new constructorFunction(arguments);

Parameters:

• ConstructorFunction: A class or function that specifies the type of the object instance.
• Arguments: A list of values that the constructor will be called with.

Return Value:

• If the constructor function returns a non-primitive value (Object, array, etc), the constructor function still returns that value. Which means the new operator won't change the returned value.
• If the constructor function returns nothing, 'this' is returned;
• If the constructor function returns a primitive value,  it will be ignored, and 'this' is returned.

Example 1: This example shows the use of a new keyword.

function Fruit(color, taste, seeds) {
    this.color = color;
    this.taste = taste;
    this.seeds = seeds;
}

// Create an object
const fruit1 = new Fruit('Yellow', 'Sweet', 1);

// Display the result
console.log(fruit1.color);

Output:

Yellow

Explanation: In the above example, the 'new' keyword creates an empty object. Here, Fruit() includes three properties 'color', 'taste', and 'seeds' that are declared with 'this' keyword. So, a new empty object will now include all these properties i.e. 'color', 'taste' and 'seeds'. The newly created objects are returned as fruit1().

Example 2: This example shows the use of a new keyword.

function func() {
    let c = 1;
    this.a = 100;
}

// Set the function prototype
func.prototype.b = 200;

// Create an object
let obj = new func();

// Display the result on console
console.log(obj.a);

console.log(obj.b);

Output:

100 
200

Explanation: In the above example, the 'new' keyword creates an empty object and then sets the 'prototype' property of this empty object to the prototype property of func(). New property 'b' is assigned using func.prototype.y. So, the new object will also include 'b' property. Then it binds all the properties and functions declared with this keyword to a new empty object. Here, func() includes only one property 'a' which is declared with this keyword. So new empty object will now include 'a' property. The func() also includes 'c' variable which is not declared with this keyword. So 'c' will not be included in new object. Lastly, the newly created object is returned. Note that func() does not include areturn statement. The compiler will implicitly insert 'return this' at the end.