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function class_exists (cls) {
    // Checks if the class exists  
    // 
    // version: 1008.1718
    // discuss at: http://phpjs.org/functions/class_exists    // +   original by: Brett Zamir (http://brett-zamir.me)
    // *     example 1: function class_a() {this.meth1 = function () {return true;}};
    // *     example 1: var instance_a = new class_a();
    // *     example 1: class_exists('class_a');
    // *     returns 1: true    var i = '';
    cls = this.window[cls]; // Note: will prevent inner classes
 
    if (typeof cls !== 'function') {return false;}
     for (i in cls.prototype) {
        return true;
    }
    for (i in cls) { // If static members exist, then consider a "class"
        if (i !== 'prototype') {            return true;
        }
    }
    if (cls.toSource && cls.toSource().match(/this\./)) { 
        // Hackish and non-standard but can probably detect if setting        // a property (we don't want to test by instantiating as that
        // may have side-effects)
        return true;
    }
        return false;
}

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function class_a() {this.meth1 = function () {return true;}};
var instance_a = new class_a();
class_exists('class_a');

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true