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copied!<p>Even though <a href="https://stackoverflow.com/questions/6039265/50-50-false-50-50-true/6039320#6039320">Jeremy Heiler</a> is correct, his justification is misconstrued. Why you're getting <code>object</code> instead of <code>number</code> has nothing to do with constructors.</p> <p>The problem here is the <code>this</code> keyword. You need to understand what happens whenever you use <code>this</code>. A little bit of digging through the ECMA draft will show you that</p> <blockquote> <p>The this keyword evaluates to the value of the ThisBinding of the current execution context.</p> </blockquote> <p>(I would change the above. The <code>this</code> keyword doesn't evaluate to the <strong>value</strong> of anything as we'll soon see.) Hmm, okay, but how exactly does <code>ThisBinding</code> work? Read on!</p> <blockquote> <p>The following steps are performed when control enters the execution context for function code contained in function object F, a caller provided thisArg, and a caller provided argumentsList:</p> <ol> <li>If the function code is strict code, set the ThisBinding to thisArg.</li> <li>Else if thisArg is null or undefined, set the ThisBinding to the global object.</li> <li><strong>Else if Type(thisArg) is not Object, set the ThisBinding to</strong> <strong>ToObject(thisArg).</strong></li> <li>Else set the ThisBinding to thisArg.</li> <li>Let localEnv be the result of calling NewDeclarativeEnvironment passing the value of the [[Scope]] internal property of F as the argument.</li> <li>Set the LexicalEnvironment to localEnv.</li> <li>Set the VariableEnvironment to localEnv.</li> <li>Let code be the value of F’s [[Code]] internal property.</li> <li>Perform Declaration Binding Instantiation using the function code code and argumentList as described in 10.5</li> </ol> </blockquote> <p>And therein lies the rub (look at the bolded part). If a function is ever called from a non-object context, <code>ThisBinding</code> (aka using <code>this</code>) always returns the value of the context wrapped inside an object. The easiest way to fix it would be to do:</p> <pre><code>Number.prototype.abs = function () { "use strict"; // if available // * 1 implicitly coerces 'this' to a number value return this >= 0 ? this * 1 : this * -1; //... or Number(this) explicitly coerces 'this' to its toNumber value return Number(this >= 0 ? this : this * -1); }; </code></pre> <p>...to coerce the <code>this</code> object (or to force strict mode). But I think it's important to understand how <code>this</code> works, and this question is a great example of that.</p>

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