Modifying the Return Value of a Constructor

November 30, 2018

3 min read

The new keyword does a few things behind the scenes to create a new instance of an object.

One of these things is to invoke the constructor of the object being used as the source of the new. The constructor is a function responsible for building a new object to the specifications desired by its developer.

#Standard Constructor Usage

When the constructor is actually called, it is bound with the context of the new object instance being created. This means that this inside of the constructor function refers to the new instance not the original object or class. By referencing this you can format the new object instance as desired.

class Standard {
  constructor() {
    this.property = 'initial_value';
  }
}

#Custom Return Constructor Usage

Another more uncommon feature of the constructor function is the usage of a return statement.

This allows you to override the normal object creation process and return a different object as a result of the new expression.

Among other things, you can use this to create a Singleton.

#Singleton Example

#Declaration

let instance;

class Singleton {
  constructor() {
    if (instance) {
      return instance;
    }
    instance = new Object();
  }
}

#Usage:

const a = new Singleton();
const b = new Singleton();
a === b; // true

#sources

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/new
• https://stackoverflow.com/questions/3350215/what-is-returned-from-a-constructor