Puzzling Javascript constructor issue

javascript

Solution

When instantiating a constructor, the first parens indicate what to use at the constructor.

So `new az.classes['test']();` will execute the expression `az.classes['test']` and use the resulting function as the constructor.

Or refactored, it does this:

// new az.classes['test']()
//                       ^ expression left of here used as constructor!
var MyConstructor = az.classes['test'];
new MyConstructor();

However, `new az.getClass('test')();` executes the expression `az.getClass` which returns your class getter function, and attempts to use that as the constructor. Then your instance tried to be executed with the `()`.

Or refactored, it does this:

// new az.getClass('test')();
//                ^ expression left of here used as constructor!
var MyConstructor = az.getClass;
var instance = new MyConstructor('test');
instance(); // obviously not gonna work.

See the difference?

You can solve this by wrapping more parens around the expression that should return the constructor:

new (az.getClass('test'))();
//                       ^ expression left of here used as constructor!

Or by saving the constructor reference to a local variable, and then using it on the next line. This is probably more sane and readable:

var MyConstructor = az.getClass('test');
new MyConstructor();
//               ^ expression left of here used as constructor!

Problem

A very puzzling problem i am having with JavaScript. look at the code below.. ``` az={ classes:{ test:function(){ this.hello=function(name){ alert('hello '+name); } } }, getClass:function(name){ return az.classes[name]; } }; var a=new az.classes['test'](); a.hello('foo'); var b= new az.getClass('test')(); b.hello();// fails !!! ``` in the code if you notice we have a class defined inside an object `az.classes`. when try to create an instance of that class by new `az.classes['test]()`, it works and `a.hello()` executes fine. but when i call the method `az.getClass('test')`, which in turn returns the same constructor, but it fails when i say `var b=new az.getClass('test');` here its saying `b` is `undefined`!! and `b.hello()` fails!! i do not understand this behaviour! what is the difference between new `az.classes['test']()` and new `az.getClass('test')`. Are they not the same thing??

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