JavaScript构造函数返回

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?

Answer 1

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); 

Answer 2

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

Answer 3

返回this值，以便呼叫者可以将呼叫链接在一起。

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