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    <p>The other answers are more or less correct, but what they're missing is that there's a difference between</p> <pre><code>Main.prototype.foo = { bar: 1 }; </code></pre> <p>and</p> <pre><code>Main.prototype.bar = 1; </code></pre> <p>In both cases, instantiating a new <code>Main</code> will create a new instance with, in its prototype chain, a property <code>foo</code> or <code>bar</code>. In both cases, the instance-level property can be redefined without affecting other instances:</p> <pre><code>function Main() {}; Main.prototype.foo = { bar: 1 }; Main.prototype.bar = 1; a = new Main(); b = new Main(); a.foo = { bar: 2 }; console.log(a.foo.bar, b.foo.bar); // 2 1 a.bar = 2; console.log(a.bar, b.bar); // 2 1 </code></pre> <p>But when you instantiate a new <code>Main</code>, the instance variable <code>foo</code> is a <em>reference</em> to a single object, <code>{bar:1}</code>, which is shared among all instances. So when you set <code>a.foo.bar</code>, you're changing the common object, not an instance variable; the instance variable is the reference <code>a.foo</code>.</p> <p>You don't have to initialize the instance property in the constructor. The standard approach would be to set <code>bar</code> directly on the prototype, i.e. <code>Main.prototype.bar = 1</code>, which would give you independent instance variables initialized to <code>1</code>. However, if you need a more complex data structure (an object, an array, or an instance of another class) on a per-instance basis, then you can't create this as a property on the prototype, because you'll be giving every instance a reference to a common object - so inside the constructor is the way to go:</p> <pre><code>function Main() { // instance-level object this.foo = { bar: 1 }; } </code></pre>
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    1. COSee this excellent post to clearly understand how prototypes work: https://javascriptweblog.wordpress.com/2010/06/07/understanding-javascript-prototypes/
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    2. COI think the statement "instance variable foo is a reference to a single object" is important to understand the behavior of manipulating an attribute within foo.
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