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POWhy does adding a property to an object literal's prototype change its "type"?
 

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  1. POWhy does adding a property to an object literal's prototype change its "type"?
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    <p>So I have a simple <code>isPlainObject</code> method that I use to test for JavaScript object literals:</p> <pre><code>var isPlainObject = function (obj) { return typeof obj === "object" &amp;&amp; {}.toString.call(obj) === "[object Object]"; }; </code></pre> <p>Now I have a plain object:</p> <pre><code>var obj = {'one': 1, 'two': 2, 'three': 3}; </code></pre> <p>When I run it through the <code>isPlainObject(obj)</code> function it works as expected and returns <code>true</code>. My question arrives from adding a property to the object's prototype:</p> <pre><code>obj.constructor.prototype.four = 4; </code></pre> <p>Now when I run <code>isPlainObject(obj)</code> on <code>obj</code> it returns <code>false</code>. <code>typeof obj</code> returns <code>object</code> in both instances. The <code>toString</code> returns <code>[object Number]</code> in the second instance after I've added a property to the prototype.</p> <p>What exactly has happened to change <code>obj</code>? What's going on?</p> <p>EDIT: This only happens when tested within the confines of a <strong>QUnit</strong> function call.</p> <pre><code>test("each", function() { _.each([1, 2, 3], function(i,v) { equal(v, i + 1, 'each iterator provided index and value from array'); }); var obj = {'one': 1, 'two': 2, 'three': 3}; console.log(_.isPlainObject(obj)); // =&gt; true obj.constructor.prototype.four = 4; console.log(_.isPlainObject(obj)); // =&gt; false }); </code></pre> <p>EDIT: This is the console.log I get when logging the <code>arguments</code> array-like object within <code>isPlainObject</code>. </p> <p><img src="https://i.stack.imgur.com/AVckV.png" alt="Logging out the &lt;code&gt;arguments&lt;/code&gt; object"></p> <p>Looking at the log would seem to indicate the array now has two arguments. But the length still reads <code>1</code>.</p>
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    1. COWhat (weird) browser are you using? [It shows `true` for me...?](http://jsfiddle.net/mattlunn/bR7PX/)
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    2. COI'm testing within a QUnit test() call. And you're right I just ran the test in the console and everything works as expected. Why would running this test within QUnit have this effect. Some sort of scope issue?
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    3. COIt's true for me, too. What do `typeof obj` and `{}.toString.call(obj)` return for you?
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      1. USBarmar
 

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