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    <p>The problem is that you can't easily create a prototype object for <code>B</code> since invoking the constructor of <code>A</code> can't be done. This is due to the parameters for the constructor being unknown before <code>new B</code> is executed. You need a dummy constructor function to construct a prototype for <code>B</code> that links to <code>A</code>'s prototype.</p> <pre><code>B.prototype = (function(parent){ function protoCreator(){}; protoCreator.prototype = parent.prototype; // Construct an object linking to A.prototype without calling constructor of A return new protoCreator(); })(A); </code></pre> <p>Once you've got the prototype object for <code>B</code> set up, you need to ensure to call the constructor of <code>A</code> in the constructor of <code>B</code>.</p> <pre><code>function B(x, y) { // Replace arguments by an array with A's arguments in case A and B differ in parameters A.apply(this, arguments); } </code></pre> <p>You should now be able to instantiate <code>B</code> by calling <code>new B(x, y)</code>.</p> <p>For a complete same including parameter validation in <code>A</code> see <a href="http://jsfiddle.net/bKUF6/" rel="nofollow">a jsFiddle</a>.</p> <p>In your original code you are setting <code>B.prototype.constructor = B</code>. I'm not getting why you are doing this. The <code>constructor</code> property does not influence the inheritance hierarchy for which the <code>prototype</code> property is responsible. If you want to have the named constructor contained in the <code>constructor</code> property you'd need to extend the code from above a little:</p> <pre><code>// Create child's prototype – Without calling A B.prototype = (function(parent, child){ function protoCreator(){ this.constructor = child.prototype.constructor }; protoCreator.prototype = parent.prototype; return new protoCreator(); })(A, B); </code></pre> <p>Using the first definition of <code>B.prototype</code> you'd get the following results:</p> <pre><code>var b = new B(4, 6); b.constructor // A console.info(b instanceof A); // true console.info(b instanceof B); // true </code></pre> <p>With the <a href="http://jsfiddle.net/bKUF6/1/" rel="nofollow">extended version</a>, you'll get:</p> <pre><code>var b = new B(4, 6); b.constructor // B console.info(b instanceof A); // true console.info(b instanceof B); // true </code></pre> <p>The cause for the different output is that <code>instanceof</code> follows up the whole prototype chain of <code>b</code> and tries to find a matching prototype object for <code>A.prototype</code> or <code>B.prototype</code> (in the other call). The <code>b.constructor</code> prototype does refers to the function that was used to define the instances prototype. In case you wonder why it does not point to <code>protoCreator</code> this is because its prototype was overwritten with <code>A.prototype</code> during the creation of <code>B.prototype</code>. The extended definition as show in <a href="http://jsfiddle.net/bKUF6/1/" rel="nofollow">the updated example</a> fixes this <code>constructor</code> property to point to a more appropriate (because probably more expected) function.</p> <p>For daily use, I'd recommend to discard the idea of using the <code>constructor</code> property of instances entirely. Instead do use <code>instanceof</code> since its results are more predictable/expected.</p>
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    1. POJavaScript inheritance: when constructor has arguments
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    1. COThank you for your answer! I think I have the same question for you. . I'm wondering why I cant do this directly without using a mediator: function A() {}; A.prototype.run = function () {}; function B() {}; B.prototype = A.prototype; B.prototype.constructor = B;
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    2. COIf you'd set `B.prototype = A.prototype` you'd loose the ability to add methods or properties to `B` instances without them being present in `A` instances. Your assignment would lead to instances of `A` and `B` sharing the same prototype – the prototype is where the properties and methods of instances come from.
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    3. COThe reason why I was using `B.prototype.constructor = B` is that, after `B.prototype = new A`, the constructor of B is actually pointing to A. This way, I wouldn't be able to extend the constructor of B. Actually your extended examples tell about this problem too.
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