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    <p>You're right that JavaScript doesn't have classes (yet), but it does have constructor functions, an <code>instanceof</code> operator that defines a relationship between objects and constructors, and a form of inheritance based on prototype chains.</p> <p><code>obj instanceof ctor</code> is true when <code>ctor.prototype</code> is on <code>obj</code>'s prototype chain.</p> <p>Modulo the caveat below, you could implement <code>instanceof</code> in EcmaScript 5 thus</p> <pre><code>function isInstanceOf(obj, ctor) { var proto = ctor.prototype; if (typeof obj === "object" || typeof obj === "function") { while (obj) { if (obj === proto) { return true; } obj = Object.getPrototypeOf(obj); } } return false; } </code></pre> <p>Unless you go around reassigning prototypes (<code>o = new MyConstructor(); MyConstructor.prototype = somethingElse</code>) it should be the case that <code>new MyConstructor() instanceof MyConstructor</code>.</p> <p>Section <a href="http://es5.github.com/#x15.3.5.3">15.3.5.3</a> explains this in detail.</p> <blockquote> <p>15.3.5.3 <code>[[HasInstance]] (V)</code></p> <p>Assume F is a Function object.</p> <p>When the [[HasInstance]] internal method of F is called with value V, the following steps are taken:</p> <ol> <li>If V is not an object, return false.</li> <li>Let O be the result of calling the [[Get]] internal method of F with property name "prototype".</li> <li>If Type(O) is not Object, throw a TypeError exception.</li> <li><p>Repeat</p> <ol> <li>Let V be the value of the [[Prototype]] internal property of V.</li> <li>If V is null, return false.</li> <li>If O and V refer to the same object, return true.</li> </ol></li> </ol> </blockquote> <p>This isn't the whole story because host objects (like DOM nodes) are allowed to implement the <code>[[HasInstance]]</code> internal method however they like but most browsers implement host objects to behave as closely to native objects as possible.</p>
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    1. POWhat does 'is an instance of' mean in Javascript?
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    1. USMike Samuel
    1. USMike Samuel
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    1. CODont says `(yet)` :( classes are still strawman. If we're lucky they wont get into ES6 !
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      1. USRaynos
 

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