Constructor and "new" operator

Constructor function

Function that is meant to be a constructor are named with capital letter first and be executed with the new operator.

We use a constructor because it creates a shorter and simpler syntax compared to creating object with {...} every time.

function User(name) {
	this.name = name;
    this.isAdmin = false;
}

// Invoking constructor
let user = new User("Ricky");
console.log(user.name);	   // Ricky
console.log(user.isAdmin); // false

When a function is executed with the new operator here it what happens

1. A new empty object is created and assigned to this implicitly
2. The function body executes, usually modifying this by adding new properties or method to it
3. Then the value of this is returned implicitly

So step 1 and step 3 is done implicitly for you already, you just need to worry about modifying this.

new function() {...}

Using this syntax you can create a single complex object that you don't aim to create again in the future. The constructor isn't saved anywhere.

let user = new function() {
	this.name = "John"
    this.isAdmin = false;
};

The constructor function is created and immediately called with the new operator.

Returning from constructor

Normally, a constructor function don't have a return statement. But if there is one the rule are as follows:

1. If return is called with an object, then the object is returned instead of this
2. If return is called with a primitive, it's ignored

Methods in constructor

You can add method in a constructor function as well, the same way you would add a property!

function User(name) {
	this.name = name;
    
    this.foo = function() {
    	console.log(`My name is ` + this.name);
    };
}

You are able to write arrow functions and refer to this in constructor function as the this is bounded to the constructor's this.

function User(name) {
    this.name = name;
    
    this.foo = () => {
        console.log(this.name);
    };
    
    this.bar = function() {
        console.log(this.name);
    }
}

let u1 = new User("Ricky");
u1.foo(); // Ricky
u1.bar(); // Ricky