arguments.callee - arguments.callee

arguments.callee

arguments.callee is a powerful JavaScript construct that allows a function to call itself. Sound useless? It's not. arguments.callee allows even anonymous functions to be recursive. A classic example of recursion is something like this:

    function getFactorial(x) {
        if (x <= 1) {
            return 1;
        } else {
            return x * arguments.callee(x - 1);
        }
    }
    alert(getFactorial(5)); // evaluates 5*4*3*2*1, alerts "120"

That particular example can be done without recursion, and in general is not very exciting. However, this blog will delve into various ways arguments.callee can be used to solve real programming problems. Another fun construct is setTimeout(arguments.callee, x) which allows a function to call itself after a delay of x milliseconds. Here's a fun little example:

(function() {
    var message = "Asynchronous output!";                            // statement 1
    var target = document.getElementById("post-example");            // statement 2
    if (target.innerHTML === message) {
        target.innerHTML = "";                                       // statement 3
    } else {
        target.innerHTML += message.charAt(target.innerHTML.length); // statement 4
    }
    setTimeout(arguments.callee, 200);                               // statement 5
})();

(function(){})(); creates an anonymous function, then immediately runs it. Handy when you want to execute code without worrying about scope issues, but that's out of the scope of this post...

The first two statements of our anonmyous function declare what we want to output, and where we want to send it. Statement 3 resets the target once the full message has been displayed, and statement 4 copies over a single character of our message.

Statement 5 is the fun part. Our unnamed function calls itself 5 times per second, outputting one letter each time. There's more to it than that, but hopefully that serves as an introduction to recustion using arguments.callee.

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