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    <p>Oh boy, you are confusing quite a few things. </p> <pre><code>function CarConstructor(){ this.speed=19; // in mph this.make="Ford"; this.fillKph; // &lt;-&gt; This particular statement has no meaning. //When you write this.fillKph without any assignment, it will be 'undefined'. //Just because you have a function named 'fillKph' somewhere else, //it doesn't mean it will get attached to this property. } </code></pre> <p>Try,</p> <pre><code>var toyota = new Car(); alert(typeof toyota.fillKph); //will alert undefined. </code></pre> <p>The fillKph function is created in global scope, i.e. as property of 'Window' object.</p> <pre><code>function fillKph(){ var me=this; $("#kphdiv").html(me.speed*1.61); } </code></pre> <p>To fix it, you can what rezzif suggested. Your final code will look like</p> <pre><code>function Car() { this.speed=19; // in mph this.make="Ford"; this.fillKph = function (){ $("#kphdiv").html(this.speed*1.61); }; } car1 = new Car(); car1.fillKph(); </code></pre> <p>If you notice, I did not store reference to 'this' inside a local variable. Why? There is no need in this scenario. To understand more, see <a href="https://stackoverflow.com/questions/1007340/javascript-function-aliasing-doesnt-seem-to-work/1162192#1162192">my detailed answer here</a>. </p> <p>If you are going to create lot of Car objects, you can define the fillKph method on the prototype.</p> <pre><code>function Car() { this.speed=19; // in mph this.make="Ford"; } Car.prototype.fillKph = function fillKph() { $("#kphdiv").html(this.speed*1.61); }; car1 = new Car(); car1.fillKph(); </code></pre> <p><strong>EDIT:</strong></p> <p>If you do something like,</p> <pre><code>function CarConstructor(){ this.speed=19; // in mph this.make="Ford"; this.fillKph = fillKph; } function fillKph(){ $("#kphdiv").html(me.speed*1.61); } car1 = new Car(); car1.fillKph(); //This will work as expected. </code></pre> <p>But the problem is that fillKph is defined in 'Window' scope, so I can directly call it like,</p> <pre><code>fillKph(); //Calling it this way will break it as it won't get correct 'this'. </code></pre> <p>Point is,</p> <pre><code>alert(typeof fillKph); // alerts 'function' if you do it your way, alert(typeof fillKph); // alerts 'undefined', if you do it the way I suggested, which is preferred in my opinion. </code></pre>
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    1. POHow does 'this' work in JavaScript?
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    1. COI fixed my this.fillKph => this.fillKph=fillKph; typo ... Is that still bad form?
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    2. COwow ... answered ... and in detail ... thanks
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    3. COTravis, I edited my answer to clarify the doubt you have raised in your comment.
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