Augment the Array type

I'm trying to augment Array as follows:

Array.prototype.myFunction = function () { return 10; }

But then, the following doesn't work:

var myArray = "Bla"

myArray.myFunction();

What's the stup开发者_如何学JAVAid mistake I'm doing?

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Because "Bla" is not an Array. Its a String.