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copied!<p><code>constructor</code> is a regular property of the prototype object (with the <code>DontEnum</code> flag set so it doesn't show up in <code>for..in</code> loops). If you replace the prototype object, the <code>constructor</code> property will be replaced as well - see <a href="http://joost.zeekat.nl/constructors-considered-mildly-confusing.html" rel="noreferrer">this explanation</a> for further details.</p> <p>You can work around the issue by manually setting <code>Obj2.prototype.constructor = Obj2</code>, but this way, the <code>DontEnum</code> flag won't be set.</p> <p>Because of these issues, it isn't a good idea to rely on <code>constructor</code> for type checking: use <code>instanceof</code> or <code>isPrototypeOf()</code> instead.</p> <hr> <p>Andrey Fedorov raised the question why <code>new</code> doesn't assign the <code>constructor</code> property to the instance object instead. I guess the reason for this is along the following lines:</p> <p>All objects created from the same constructor function share the constructor property, and shared properties reside in the prototype.</p> <p>The real problem is that JavaScript has no built-in support for inheritance hierarchies. There are several ways around the issue (yours is one of these), another one more 'in the spirit' of JavaScript would be the following:</p> <pre><code>function addOwnProperties(obj /*, ...*/) { for(var i = 1; i < arguments.length; ++i) { var current = arguments[i]; for(var prop in current) { if(current.hasOwnProperty(prop)) obj[prop] = current[prop]; } } } function Obj1(arg1) { this.prop1 = arg1 || 1; } Obj1.prototype.method1 = function() {}; function Obj2(arg1, arg2) { Obj1.call(this, arg1); this.test2 = arg2 || 2; } addOwnProperties(Obj2.prototype, Obj1.prototype); Obj2.prototype.method2 = function() {}; </code></pre> <p>This makes multiple-inheritance trivial as well.</p>

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