John Resig’s Inheritance

Ciprian on Tuesday, January 16, 2018 in Methods, Events and Scopes
Last modified on Tuesday, May 10, 2022

John Resig, the creator of jQuery, has created this JavaScript inheritance code. I am still using it now to simplify several aspects of my Hound CMS.

/* Simple JavaScript Inheritance
 * By John Resig https://johnresig.com/
 * MIT Licensed.
 */
// Inspired by base2 and Prototype
(function() {
    var initializing = false,
    fnTest = /xyz/.test(function() {xyz;}) ? /\b_super\b/ : /.*/;

    // The base Class implementation (does nothing)
    this.Class = function(){};

    // Create a new Class that inherits from this class
    Class.extend = function(prop) {
        var _super = this.prototype;

        // Instantiate a base class (but only create the instance, don't run the init constructor)
        initializing = true;
        var prototype = new this();
        initializing = false;

        // Copy the properties over onto the new prototype
        for (var name in prop) {
            // Check if we're overwriting an existing function
            prototype[name] = typeof prop[name] == "function" && typeof _super[name] == "function" && fnTest.test(prop[name]) ? (function(name, fn) {
                return function() {
                    var tmp = this._super;

                    // Add a new ._super() method that is the same method but on the super-class
                    this._super = _super[name];

                    // The method only need to be bound temporarily, so we remove it when we're done executing
                    var ret = fn.apply(this, arguments);
                    this._super = tmp;

                    return ret;
                };
            })(name, prop[name]) : prop[name];
        }

        // The dummy class constructor
        function Class() {
            // All construction is actually done in the init method
            if (!initializing && this.init) {
                this.init.apply(this, arguments);
            }
        }

        // Populate our constructed prototype object
        Class.prototype = prototype;

        // Enforce the constructor to be what we expect
        Class.constructor = Class;

        // And make this class extendable
        Class.extend = arguments.callee;

        return Class;
    };
})();

Usage:

var Person = Class.extend({
    init: function(isDancing) {
        this.dancing = isDancing;
    },
    dance: function() {
        return this.dancing;
    }
});

var Ninja = Person.extend({
    init: function() {
        this._super(false);
    },
    dance: function() {
        // Call the inherited version of dance()
        return this._super();
    },
    swingSword: function() {
        return true;
    }
});

var p = new Person(true);
p.dance(); // => true

var n = new Ninja();
n.dance(); // => false
n.swingSword(); // => true

// Should all be true
p instanceof Person && p instanceof Class &&
n instanceof Ninja && n instanceof Person && n instanceof Class

The original article, complete with explanations and examples is here: https://johnresig.com/blog/simple-javascript-inheritance/.

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