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POJavascript: z = z || [] throws an error when not using VAR - why?
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  1. POJavascript: z = z || [] throws an error when not using VAR - why?
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    copied!<p>Out of just intellectual curiosity, why does javascript accept </p> <pre><code>var z = z || []; </code></pre> <p>to initialize z (as z may defined initially)</p> <p>but without var, it throws an error (in global space)</p> <pre><code>z = z || []; </code></pre> <p>(if z is previously undefined)</p> <p>In the global space you are not required to use VAR though I get it might be bad practice.</p> <p>Before you say this is a duplicate of questions like</p> <p><a href="https://stackoverflow.com/questions/1470488/difference-between-using-var-and-not-using-var-in-javascript">What is the purpose of the var keyword and when to use it (or omit it)?</a></p> <p>Note the declaration that <strong><em>"If you're in the global scope then there's no difference."</em></strong></p> <p>Obviously that's not 100% true, given my working example.</p> <p>Is this a quirk or is there legitimate logic?</p> <hr> <p>adding a summary of the answer as I've learned it:</p> <p>Thanks to Tim (see below) the key to my misunderstanding was not realizing this (fundamental of javascript) </p> <blockquote> <p>var z; does absolutely nothing if z already exists</p> </blockquote> <p>That's how this expression seems to have it both ways, if you incorrect assume that "var z" always initializes.</p> <p>Starting from the left, "var z" simply makes sure z is defined but does not actually affect any existing value if it already exists. Then on the right, if z already exists, it is used, if not, the variable was just declared (but empty) so it won't be used but will not throw an error.</p> <p>This is an excellent article on this kind of scoping and hoisting issue in Javascript: <a href="http://www.adequatelygood.com/2010/2/JavaScript-Scoping-and-Hoisting" rel="nofollow noreferrer">http://www.adequatelygood.com/2010/2/JavaScript-Scoping-and-Hoisting</a></p> <p>Many thanks to minitech and everyone else who contributed too!</p>
 

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