Unexpected Javascript for-loop behavior inside Chrome

Question

So I am getting incredible weird java script behavior inside a chrome tab. The page is behind a login so I can't post it but can someone explain exactly what is happening???

for(var z in ""){ console.log(z) }
contains
//undefined

hmm...

var key = ""

for(var i in key){ console.log(i) }
contains
//undefined
Object.getOwnPropertyNames(key)
//["length"]
Object.getOwnPropertySymbols(key)
//[]
window[key]
//undefined

At first I thought this was one of those JS behaviors and was ready to submit it to JSWTF but the behavior runs properly in another chrome tab:

for(var i in ""){ console.log('ran',i) }
//undefined

How did a value get assigned to a blank string? Where is it? What is the for loop doing?

edit: The same page in firefox returns expected behavior in console. I have not tested other browsers

Answer 1

You have an ES6 shim on the original page which adds the function contains() to the String prototype. You can do this yourself by doing something like:

String.prototype.contains = 
    function(e) {
        return this.indexOf(e) > -1;
    };

The ES6 function ultimately standardized on is includes() , so you'll probably see that function name change in the future when a developer updates the shim.