Chrome Error-Uncought TypeError:string is not a function

Occurs when you assign a same name to function as of variable in js

elements have a download attribute in HTML5 as explained here, with a default value of "" (an empty string).
This means that download === this.download in the onclick handler (this is the element in onevent attributes), and therefore the download attribute of the element is superior to the download property of window.
This fiddle lists all string attributes that are present by default. You can see download is an attribute just like innerHTML, which also fails with the exact same reason when used as a function (i.e. trying to refer to window.innerHTML, but instead executing elem.innerHTML()).
As said in the comments, using window makes for no confusion as to what property/attribute variables will evaluate to.
 

This scope behaviour does actually not seem to due to the this value but rather a specific "scope chain" that is being constructed. As per the HTML5 specification:

Lexical Environment Scope
Let Scope be the result of NewObjectEnvironment(the element's Document, the global environment).
If the element has a form owner, let Scope be the result of NewObjectEnvironment(the element's form owner, Scope).
Let Scope be the result of NewObjectEnvironment(the element's object, Scope).

I.e. what is happening is the scope chain is window -> document -> element (increasing superiority). This means that download evaluates to element.download and not window.download. What also can be concluded from this is that getElementById will bubble up to document.getElementById (given elem.getElementById does not exist).
I set up a systematic example so that you can see how variables bubble up the scope chain:

window.a   = 1;
document.a = 2;
elem.a     = 3;

window.b   = 4;
document.b = 5;

window.c   = 6;

Then, logs 3, 5, 6 when clicked.