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On HTML5 Boilerplate they use this code for jQuery:

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
<script>window.jQuery || document.write('<script src="js/libs/jquery-...

What's more readable:

if (!window.jQuery) document.write(...);

or

window.jQuery || document.write(...);
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    Another thing we sometimes do in H5BP is introduce clever tricks like this that lead to a better understanding of code. I think this is a good example. The if statement is almost TOO obvious; it can afford to teach you something at the same time. Commented Apr 10, 2012 at 4:40
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    Any javascript programmer worth their salt will be able to read both forms equally well. When it comes to inline code, I typically prefer the shortest reasonable form, so I would use window.jQuery||document.write... for sake of brevity. Commented Apr 10, 2012 at 14:41
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    @PaulIrish I can't tell whether it's sarcasm or not. Commented Apr 10, 2012 at 19:41
  • So the idea is to teach a trick. -legitimate- but H5BP puts a stamp on everything saying essentially this ok, often probably being interpreted as the recommended way. (For H5BP) it's a matter of what's most valued - I say K.I.S.S. I vouch for stupid simple code that is really obvious. Either way developers are learning how to do a local fallback, eliminating clutter around how to do that I think would be best. Local fallback is a good trick. Short circuit statements, bad trick. Teach only good tricks? Commented Apr 10, 2012 at 19:43
  • Doesn't directly address this, but see javascript.crockford.com/style2.html Commented Jan 16, 2013 at 22:30

4 Answers 4

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I prefer 

if (!window.jQuery) 
    document.write( -local jQuery- );

because it makes it obvious that the document.write() depends on some condition. With the second option, you are abusing short-circuit evaluation, so most developers will have to think harder to understand what you are doing.

I generally don't expect to see code that creates output to be used with a boolean operator like that.

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    This code will throw an error without the presence of window. in the if condition. See my comment on the question as for why I think it's okay our logic is backwards. Commented Apr 10, 2012 at 17:04
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    Use brackets you whitespace devil Commented Apr 10, 2012 at 19:41
  • Also noticed we can flip the boolean statement logic to be !window.jQuery && document.write( -local jQuery- ); Commented Apr 10, 2012 at 19:57
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    @DevinGRhode The final two characters in that code line seem surprisingly appropriate... Commented Apr 10, 2012 at 20:15
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The if-statement is a statement and as such does not have a result. The or-expression has a result which is immediately thrown away. Therefore, the if-statement is less complex and should be preferred.

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    Not so. In this idiom, the short-circuiting term (e.g., window.jQuery) is used, negated, as the if condition. There is no reason to choose one over the other except style preferences. Commented Apr 10, 2012 at 18:23
  • Semantically, there is no difference. However, the expression '!a || b' has a type (boolean), whereas 'if (a) b' has no type (or void). Therefore, the expression '!a || b' could be seen as having more grammatical complexity. Then again, I've made and seen languages where 'if' constructs are expressions themselves. Commented Apr 11, 2012 at 6:33
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It is a shell idiom imported in JavaScript (perhaps through perl where it is also idiomatic). If you have a team used to it, it doesn't hinter the readability (some may even find it is an improvement), but I'm far from sure that it is known by people without a perl or shell background.

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    I've never been at all keen on associating “readability” with either Perl or Shell. (“Gesundheit” OTOH…) Commented Apr 10, 2012 at 14:01
  • @DonalFellows, in general yes. For that use of || and &&, I don't mind. Perhaps it is the contrast with the rest of the features of those languages which make it so :-) Commented Apr 10, 2012 at 14:02
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As with others I think the if version is more readable, I certainly wouldn't want to see logic like that strewn about a program. Having said that I think there is one idiomatic usage which is common in Perl that I personally rather like: when checking for a fatal return value, e.g. open(filename) || die. It seems to me this is a clear, compact way of expressing it that keeps the unlikely check out of the main logic of the code.

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