JavaScript 'is not a function' vs. 'expected function'?

Question

In the current Chrome, if I do this:

var i = 'foo';
i();

I get an error 'string is not a function'. I get similar errors if i is a number, undefined, etc.

However, from some real-life, more complex code, sometimes I see a different error:

'expected function: function(){}'

I am trying to figure out exactly how these two errors differ, or, to look at it another way, how to write a minimal code snip that will trigger the 'expected function' error.

I tried fiddling with callbacks, and call/apply, but none of those trigger this. Can anyone explain how to reproduce this error?

• Ian: Do you get both messages in the same browser? I wouldn't be surprised if it's just messages differing by browser for the same error
• user1106925: Are you using a code library? The error could be a custom error coming from there. throw new Error("expected function: function(){}")
• Ian: For example: stackoverflow.com/questions/6697947/… - looks like the "expected" message is old IE maybe?
• adeneo: @Ian - Yup, testet, chrome gives "string is not a function", while IE gives the "expected function" error on the same code examples. Never seen an "expected function" error in chrome myself ?
• GregT: Oops, I found it, not in jQuery, but one of our in-house libraries. I can't believe I didn't think to check there before posting. Thanks a lot for responding.

Answer 1

There is no specification what error message should be. Therefore each vendor implements it's own. The only way to unify this is to verify data yourslef and throw error that you expect.

var i = 'foo';
if (!$.isFunction(i)){
 throw 'expected function: function(){}';
}