为什么在使用window变量查找索引时出现错误？

Question

This is very strange, I'm trying to use a variable which I define as is:

window.params['variable'] = { ... };

In the function I'm using I have this:

function q()
{
 ...
 // for example
 return new Chart( canvas, window.params['variable'] );
 ...
}

When I execute this I get the error: Uncaught TypeError: Cannot read property 'variable' of undefined

But, incredible (for me), when I define params as:

window.params = { ... }

The error disappear, and the function runs fine! why?

(Obviusly I replace the window.params['variable'] by window.params first, but I believe that this is equivalent)

Thanks.

Answer 1

You need to define params first before adding properties to params . So the right way to do it is

window.params = {};
window.params['variable'] = { };

or

window.params = {
  'variable' : {}
};

and then

function q(){
 ...
 // for example
 return new Chart( canvas, window.params['variable'] );
 ...
}

You will get more information about this here : https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Operators/Object_initializer