Question

Here’s code from https://github.com/Khan/khan-exercises/blob/master/khan-exercise.js

for ( var i = 0; i < loading; i++ ) (function( mod ) {
    if ( !testMode && mod.src.indexOf("/khan-exercises/") === 0 && mod.src.indexOf("/MathJax/") === -1 ) {
        // Don't bother loading khan-exercises content in production
        // mode, this content is already packaged up and available
        // (*unless* it's MathJax, which is silly still needs to be loaded)
        loaded++;
        return;
    }

    // Adapted from jQuery getScript (ajax/script.js)
    var script = document.createElement("script");
    script.async = "async";

    for ( var prop in mod ) {
        script[ prop ] = mod[ prop ];
    }

    script.onerror = function() {
        // No error in IE, but this is mostly for debugging during development so it's probably okay
        // http://stackoverflow.com/questions/2027849/how-to-trigger-script-onerror-in-internet-explorer
        Khan.error( "Error loading script " + script.src );
    };

    script.onload = script.onreadystatechange = function() {
        if ( !script.readyState || ( /loaded|complete/ ).test( script.readyState ) ) {
            // Handle memory leak in IE
            script.onload = script.onreadystatechange = null;

            // Remove the script
            if ( script.parentNode ) {
                script.parentNode.removeChild( script );
            }

            // Dereference the script
            script = undefined;

            runCallback();
        }
    };

    head.appendChild(script);
})( urls[i] );

Strange thing: instead of usual for loop code block we see self invoking function!(inside of other self invoking function) Why is that? how will this function run?

Answer 1

Basically the for loop runs the function each time with value url[i] passed into the mod parameter.

for ( var i = 0; i < loading; i++ ) (function( mod ) {...The code...})(urls[i]);

if you notice in the code you will see this

(function( mod ) {...The code...})(urls[i])

which is a function call passing urls[i] in to the parameter mod