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    <p>JavaSript does not have true multiple inheritance. You can inherit from only one prototype and then copy the rest of the properties that you want. You can test this by using the <code>instanceof</code> operator.</p> <p>After fixing the mis-spellings, your demo works, but in actuality, you are not truly inheriting. To do true JS inheritance:</p> <pre><code>function A(){} function B(){} B.prototype = new A; b = new B; console.log(b instanceof A, b instanceof B); //-&gt; true, true </code></pre> <p><a href="https://stackoverflow.com/questions/6887828/does-javascript-support-multiple-inheritance-like-c">See also</a></p> <p>More about <a href="https://developer.mozilla.org/en-US/docs/JavaScript/Introduction_to_Object-Oriented_JavaScript#Inheritance" rel="nofollow noreferrer">JS inheriance on MDN</a></p> <h2>Quasi-Multiple Inheritance</h2> <pre><code>function ctorX() { this.messageX = "this is X Message"; this.alertX = function() { console.log(this.messageX); }; } function ctorY() { this.messageY = "this is Y Message"; this.alertY = function() { console.log(this.messageY); }; } function ctorZ() { ctorX.call(this); // This is the quasi-multiple inheritance this.messageZ = "this is Z Message"; this.alertZ = function() { console.log(this.messageZ); }; } ctorZ.prototype = new ctorY; // This is the actual inheritance var objz = new ctorZ(); objz.alertZ(); objz.alertY(); objz.alertX(); console.assert(objz instanceof ctorZ, 'objz is not instance of ctorZ'); console.assert(objz instanceof ctorY, 'objz is not instance of ctorY'); console.assert(objz instanceof ctorX, 'objz is not instance of ctorX'); //The last assert will fail since there is no true multiple inheritance </code></pre> <h3><a href="http://jsfiddle.net/kaleb/ZmHLm/" rel="nofollow noreferrer">Demo of Quasi-Multiple Inheritance</a></h3> <h2>Avoid Calling the Super Constructor</h2> <p><a href="https://stackoverflow.com/users/1641941/hmr">HMR</a> brought up the point that in some instances, a user wants to inherit from a particular constructor, but the super-constructor requires parameters and will fail w/o them. The way to bypass this is to create a proxy constructor:</p> <pre><code>function C(x){if(!x) throw new Error;} function D(){} function proxyCtor(){/*should be noop*/} proxyCtor.prototype = C.prototype; D.prototype = new proxyCtor; var d = new D; console.assert(d instanceof C, 'c is not instance of D'); // will err if incorrect, which it's not </code></pre> <h3><a href="http://jsfiddle.net/kaleb/ZmHLm/3/" rel="nofollow noreferrer">Demo</a></h3>
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    1. POJavascript multiple inheritance
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    1. COI'm sorry but what's the point of the same thing twice in `log`?
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    2. COPlease, do not promote the `Subclass.prototype = new Class();` pattern. It's ugly and inefficient.
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    3. COIt is the only way for the `instanceof` operator to work. In that case any inefficiency would be negligible if the whole point is to have true inheritance. Also, please don't promote the usage of the word _Class_ when describing JavaScript constructors.
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