CODEWITHSUNDEEP
javascriptfor-loopself-invoking-function
DrStrangeLove
DrStrangeLove

Reputation: 11567

self invoking function in for loop

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?

Upvotes: 0

Views: 3773

Answers (2)

James Montagne
James Montagne

Reputation: 78740

It's an odd construct, but basically if you exclude the {} from a for loop, it will simply run the next line for each iteration, it's similar to excluding {} with an if if you want a one-line if.

So it's basically equivalent to this:

function doSomething(){...}

for ( var i = 0; i < loading; i++ ) {
   doSomething(urls[i]);
}

Where doSomething is that large function.

Upvotes: 1

John Hartsock
John Hartsock

Reputation: 86892

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

Upvotes: 3

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