Question

I thought scope chain would make the first “test = new test();” work, but it doesn’t. why?

var tool = new Tool();
tool.init();

function Tool(){
    var test;

    function init(){
        //does not work, undefined
        test = new Test();

        //does work
        this.test=new Test();

        console.log(test);
    }
}

function Test(){
}

EDIT: by not working i mean, it says that test is ‘undefined’

Answer 1

It’s simple. Your Tool instance does not have a init method. The init function in your code is merely a local function of the Tool constructor. Tool instances do not inherit such functions.

If you want your Tool instances to have a init method, you can:

1. assign it as a method inside the constructor:

   function Tool () {
       this.init = function () { ... };
   }
   

2. or assign it to Tool.prototype (outside of the constructor!):

   Tool.prototype.init = function () { ... };
   

The second option performs better, since all instances share the same init function. (In the first option, each instance gets its own init function which is created during the constructor call.)