code snippet for callback function in javascript

Recently, I've been working on a project with angularjs and phonegap and stumbled upon this interesting code snippet. While I have a basic understanding of what it does, the inner workings are still a bit unclear to me. Since I'm still getting familiar with advanced javascript concepts, I would greatly appreciate if someone could provide a more detailed explanation.

Here are my questions:

  1. Within the callback registration, I noticed that the variables "arguments" (line 5) and "fn" (line 10) are not explicitly defined anywhere in the function. Are they declared within the function prototype?

  2. Regarding the code blocks on lines 9 and 15, are they simply setting the value of "this" so that it points to the same object within the callback? What is the term used for this technique of applying the "this" value?


myApp.factory('phonegapReady', function() {
    return function (fn) {
        var queue = [];
        var impl = function () {
            queue.push(Array.prototype.slice.call(arguments));    
        };

        document.addEventListener('deviceready', function () {
            queue.forEach(function (args) {
                fn.apply(this, args);
            });
            impl = fn;
        }, false);

        return function () {
            return impl.apply(this, arguments);
        };
    };
});

Answer №1

Arguments serves as a variable that remains accessible within a function, containing all the arguments passed to it in an array-like object form. The utilization of Array.prototype.slice.call is necessary because arguments does not directly act as an array but rather as an object with incremental numeric keys like 0, 1, etc., and this code transforms it into a proper JavaScript Array.

The parameter fn is received by the function at line 2, making it accessible to the inner functions at lines 10 and 12 thanks to closures (allowing the inner function to access all variables from its outer function even after it has returned).

Regarding the usage of apply calls alongside this, you are correct in understanding that it determines the object on which the function is invoked, consequently influencing what this points to within those functions.

Nevertheless, it's crucial to understand that the value of this is dynamic and established when a function is executed, not during its declaration. For instance, within the forEach loop, this refers to the global object instead of the object triggering the deviceready event - something to keep in mind. In the final line, the determination of this will vary based on how the outcome of the entire phonegapReady callback is triggered.

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