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jQuery.fn.decodeEffect = (function ($) {
    var defaultOptions = {
        duration:      3000,
        stepsPerGlyph: 10,
        codeGlyphs:    "ABCDEFGHIJKLMNOPQRSTUWVXYZ1234567890",
        className:     "code"
    };
    
    // get a random string from the given set,
    // or from the 33 - 125 ASCII range
    function randomString(set, length) {
        var string = "", i, glyph;
        for(i = 0 ; i < length ; i++) {
            glyph = Math.random() * set.length;
            string += set[glyph | 0];
        }
        return string;
    }
    
    // this function starts the animation. Basically a closure
    // over the relevant vars. It creates a new separate span
    // for the code text, and a stepper function that performs
    // the animation itself
    function animate(element, options) {
        var text = element.text(),
            span = $("<span/>").addClass(options.className).insertAfter(element),
            interval = options.duration / (text.length * options.stepsPerGlyph),
            step = 0,
            length = 0,
            stepper = function () {
                if(++step % options.stepsPerGlyph === 0) {
                    length++;
                    element.text(text.slice(0, length));
                }
                if(length <= text.length) {
                    span.text(randomString(options.codeGlyphs, text.length - length));
                    setTimeout(stepper, interval);
                } else {
                    span.remove();
                }
            };
        element.text("");
        stepper();
    }
    
    // Basic jQuery plugin pattern
    return function (options) {
        options = $.extend({}, defaultOptions, (options || {}));
        return this.each(function () {
            animate($(this), options);
        });
    };
}(jQuery));

$("#sometext").decodeEffect();

<span id="sometext">Hello, world</span>

body { font-family: monospace; }
.code { color: red; }