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copied!<p>Try this:</p> <pre><code>function TestEngine() { this.id='Foo'; } TestEngine.prototype.fooBar = function() { this.id='bar'; return true; } TestEngine.prototype.start = function() { this.fooBar(); } TestEngine.prototype.startMethod = function() { var self = this; var inter = setInterval(function() { self.start(); }, 200); } var test = new TestEngine(); test.startMethod(); </code></pre> <p><code>setInterval</code> calls <code>start</code> function with window context. It means when <code>start</code> gets executed, <code>this</code> inside <code>start</code> function points to <code>window</code> object. And window object don't have any method called <code>fooBar</code> & you get the error. </p> <h2>Anonymous function approach:</h2> <p>It is a good practice to pass <a href="https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CC4QFjAA&url=http://helephant.com/2008/08/23/javascript-anonymous-functions/&ei=TlJIUd_wI46KrgeDiYCQCg&usg=AFQjCNEoMND3ig9KdKF1tQrR7gsp6sSAjQ&bvm=bv.43828540,d.bmk&cad=rja" rel="nofollow"><code>anonymous function</code></a> to <code>setInterval</code> and call your function from it. This will be useful if your function makes use of <code>this</code>.</p> <p>What I did is, created a temp variable <code>self</code> & assigned <code>this</code> to it when it is pointing your TestEngine instance & calling <code>self.start()</code> function with it. </p> <p>Now inside <code>start</code> function, <code>this</code> will be pointing to your testInstance & everything will work as expected. </p> <h2>Bind approach:</h2> <p><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind" rel="nofollow"><code>Bind</code></a> will make your life easier & also increase readability of your code. </p> <pre><code>TestEngine.prototype.startMethod = function() { setInterval(this.start.bind(this), 200); } </code></pre>

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