Why is the function called? JavaScript / Window

Question

I have the following code in my HTML file:

    <script type="text/javascript">
        window.never = function() {
                 console.log('this function is never called');
        }
        (function(d, s, id){
            var js, srjs = d.getElementsByTagName(s)[0];
            if (d.getElementById(id)) {return;}
            js = d.createElement(s); js.id = id;
            js.src = "this.script.does.not.exist.js";
            srjs.parentNode.insertBefore(js, srjs);
        }(document, 'script', 'streamrail-jssdk'));
    </script>

See fiddle: http://jsfiddle.net/sebvaeja/

Looking at the console, you can see that window.never function is actually called ('this function is never called' is written to the console).

When debugging this with Chrome dev tools, I see in the call stack that the caller was the closure (first line: http://jsfiddle.net/sebvaeja/).

If I change the never function to be off the global scope:

    function never() {
             console.log('this function is never called');
    }

Then it is not being called.

Can someone please explain why is window.never function being called? What is triggering the call? I guess it's got something to do with the function being on the window object, but I can't see the reasoning behind that.

Answer 1

The function expression is followed by parenthesis:

 window.never = function() { ... }
 (...)

The line break after the function expression does not terminate the variable statement, so for the parser that's a function call:

function() { ... }(...)

In fact, you are using the very same technique here:

(function(d, s, id){
  // ...
}(document, 'script', 'streamrail-jssdk'))

That's a function expression followed by (...) and it calls the function.

Solution: Add a semicolon after the definition and you are good.

---

If I change the never function to be off the global scope ... Then it is not being called.

In that case the function definition is interpreted as function declaration, not expression. A function declaration is more like a statement and therefore cannot be part of a CallExpression. The following parenthesis are therefore interpreted as grouping operator (like you intended).

Answer 2

Place the semi-colon after the function declaration:

    window.never = function() {
             console.log('this function is never called');
    };

It's because of the (...) directly afterwards that triggers the function call.

    window.never = function() {
             console.log('this function is never called');
    }
    ( ... ) // <-- Triggers call of `window.never`