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PORemoving the need for "new"
 

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  1. PORemoving the need for "new"
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    <p>A nasty gotcha in javascript is forgetting to call <code>new</code> on a function meant to be instantiated, leading to <code>this</code> being bound to a different object (usually the global) instead of a fresh one. One workaround I read about is to check for it explicitly in the function-constructor using the following idiom:</p> <pre><code>function SomeConstructor(x, y, ...) { // check if `this` is created with new if ( !(this instanceof arguments.callee) ) return new SomeConstructor(x, y, ...); // normal initialization code follows </code></pre> <p>Now <code>new SomeConstructor(...)</code> and <code>SomeConstructor(...)</code> are equivalent.</p> <p>I'd like to simplify this by creating a wrapper function <code>factory(fn)</code> that does the first two lines and then delegates to the wrapped function <code>fn</code>. This would be used like:</p> <pre><code>SomeConstructor = factory(function (x, y, ...) { // normal initialization code follows }) </code></pre> <p>My first attempt was:</p> <pre><code>function factory(fn) { return function() { if ( !(this instanceof arguments.callee) ) { return new arguments.callee.apply(this, arguments); } fn.apply(this, arguments); } } </code></pre> <p>but it fails with "Function.prototype.apply called on incompatible [object Object]". The second attempt was:</p> <pre><code>function factory(fn) { return function() { if ( !(this instanceof arguments.callee) ) { var tmp = new arguments.callee(); arguments.callee.apply(tmp, arguments); return tmp; } fn.apply(this, arguments); } } </code></pre> <p>This sort of works but it may call the wrapped function twice: once with no arguments (to create a new instance) and once with the passed arguments for the actual initialization. Apparently this is fragile and inefficient but I can't figure out a way to do it with a single call. Is this possible ?</p> <p>EDIT: Based on bobince's approach, here's a similar one that does the trick:</p> <pre><code>function factory(init) { var run_init = true; function constr() { if ( !(this instanceof constr) ) { run_init = false; var tmp = new constr(); run_init = true; init.apply(tmp, arguments); return tmp; } if (run_init) init.apply(this, arguments); } return constr; } </code></pre> <p>As for whether this is something that should be encouraged or not, that's debatable. I come from a Python background and I think of <code>new</code> as just noise (Java) or wart (Javascript), but I may be missing something.</p>
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    1. COI would recommend not trying to eliminate the use of a language construct simply because you might accidentally forget to include it. Seems like an awful lot of work just to keep from having to type three characters anyway.
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    2. CO@Darrell: This isn't so much work avoidance as it is an attempt to use metaprogramming to make everybody's life easier. This is very much the norm in dynamic languages like Javascript, Ruby, et al.
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    3. CO@John I understand and I'm all for making life easier but I still don't see how all that code is easier than typing 'new'. :)
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