神刀安全网

Return Values with the ‘new’ Operator

You’re going to run into some instances where you’ll be using new to allocate new objects in JavaScript. It’s going to blow your mind unless you read this tip to understand what’s happening behind the scenes.

The new operator in JavaScript is an operator that, under reasonable circumstances, return a new instance of an object. Let’s say we have a constructor function:

function Thing() {   this.one = 1;   this.two = 2; }  var myThing = new Thing();  myThing.one // 1 myThing.two // 2 

Note: this refers to the new object created by new . Otherwise if Thing() is called without new , no object is created , and this is going to point to the global object, which is window . This means that:

  1. You’ll suddenly have two new global variables named one and two .
  2. myThing is now undefined, since nothing is returned in Thing() .

Now that you get that example, here’s where things get a little bit wonky. Let’s say I add something to the constructor function, a little SPICE:

function Thing() {   this.one = 1;   this.two = 2;    return 5; }  var myThing = new Thing(); 

Now, what does myThing equal? Is it 5? is it an object? Is it my crippled sense of self-worth? The world may never know!

Except the world does know:

myThing.one // 1 myThing.two // 2 

Interestingly enough, we never actually see the five that we supposedly ‘returned’ from our constructor. That’s weird, isn’t it? What are you doing function? WHERE’S THE FIVE? Let’s try it with something else.

Let’s return a non-primitive type instead, something like an object.

function Thing() {   this.one = 1;   this.two = 2;    return {     three: 3,     four: 4   }; }  var myThing = new Thing(); 

Let’s check it out. A quick console.log reveals all:

console.log(myThing); /*   Object {three: 3, four: 4}   What happened to this.one and this.two!?   They've been stomped, my friend. */ 

Here’s where we learn:When you invoke a function with the new keyword, you can set properties on it using the keyword this (but you probably already knew that). Returning a primitive value from a function you called with the new keyword will not return the value you specified, but instead will return the this instance of the function (the one you put properties on, like this.one = 1; ).

However, returning a non-primitive, like an object , array , or function will stomp on the this instance, and return that non-primitive instead, effectively ruining all the hard work you did assigning everything to this .

转载本站任何文章请注明:转载至神刀安全网,谢谢神刀安全网 » Return Values with the ‘new’ Operator

分享到:更多 ()

评论 抢沙发

  • 昵称 (必填)
  • 邮箱 (必填)
  • 网址
分享按钮