Javascript constructor function return this

Question

I'm used to a javascript object constructors looking like this

function person(first, last) {
   this.firstName = first;
   this.lastName = last;
}

var dude = new person("the", "dude");

But sometimes I see the constructor return "this", like so

function person(first, last) {
   this.firstName = first;
   this.lastName = last;
   return this;
}

What's up with returning this at the end?

Please go to the link: http://stackoverflow.com/questions/8300844/what-does-return-this-do-within-a-javascript-function — Alex Repeckiy, Dec 09 '15 at 22:17

score 3 · Accepted Answer · answered Dec 09 '15 at 22:02

3

There's no point in returning this from a constructor, but you are allowed to return any object you'd like. If no object is explicitly returned, this is implicitly returned.

A possible use case is:

function person(first, last) {
  if(!(this instanceof person)) {
    return new person(first, last);
  }
  this.first = first;
  this.last = last;
}

var person1 = new person('first', 'person');
var person2 = person('second', 'person'); // <- without "new"

console.log(person1, person2);

answered Dec 09 '15 at 22:02

Amit

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Good example, I would still stay away from it :); I posted about the ["forgetting to call `new`" problem](http://js-bits.blogspot.com/2010/08/constructors-without-using-new.html) for anyone that wants further explanation as to what's going on with this question. – Ruan Mendes Dec 09 '15 at 22:10

score 0 · Answer 2 · answered Dec 09 '15 at 22:03

The only case that I know it could mean something is if you instantiate your objects in the following manner;

person.call(Object.create(person.prototype), 'Jack', 'Brown');

The above example requires that you return this;

What can be done, but should be avoided, is to return a different object than this

function person(first, last) {
    return {first: first, last: last};
}
(new Person() ) instanceof Person // false

score 0 · Answer 3 · answered Dec 09 '15 at 22:24

0

this is returned so the caller can chain calls together.

console.log(new person('First', 'Last').firstName);

answered Dec 09 '15 at 22:24

DaveB

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Javascript constructor function return this

3 Answers3