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copied!<p>There is a difference between <em>assigning</em> values and <em>referencing</em> them.</p> <pre><code>u.a = 1; </code></pre> <p>will create a new <code>a</code> property on the object referred to by <code>u</code>. Before the assignment, <code>u.a</code> will refer to <code>TestModel.prototype.a</code>, but assigning a new value actually creates a new property on the actual object:</p> <p><img src="https://i.stack.imgur.com/MoOUP.png" alt="enter image description here"></p> <p>After the assignment:</p> <p><img src="https://i.stack.imgur.com/tcROi.png" alt="enter image description here"></p> <p>On the other hand,</p> <pre><code>u.b.push(1); </code></pre> <p>will <em>not</em> create a new property. It will reference the existing property, the array, which is <code>TestModel.prototype.b</code>.</p> <blockquote> <p>even though each instance of <code>TestModel</code> should have its own instance variables according to how I've setup the prototype</p> </blockquote> <p>All instances reference the same prototype, hence they reference the same properties the prototype has. You can easily see that because <code>TestMode.prototype === u.b</code>, <code>TestMode.prototype === u2.b</code> and <code>u.b === u2.b</code> all yield <code>true</code>. </p> <p>It would work if you assign a new value as well to <code>u.b</code> and <code>u2.b</code> as well:</p> <pre><code>u.b = []; </code></pre> <p>which is normally done in the constructor:</p> <pre><code>function TestModel() { this.b = []; } </code></pre>

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