I just saw someone use this piece of code:
ctx = canvas.getContext && canvas.getContext('2d');
How does the double ampersand work in this context? Would it not just assign "true" to the ctx variable?
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Did you try some possible usecases??? But no, although it might, depending on what the function returns, if it exists.– Dave NewtonCommented Oct 14, 2012 at 1:47
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You might be interested in: stackoverflow.com/questions/3826473/…, it's Python but it applies to JS (and few other languages) as well.– NullUserExceptionCommented Oct 14, 2012 at 2:01
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2 Answers
This is a common way to make sure your function exists before you call it.
It works like this (From developer.mozilla.com):
expr1 && expr2Returnsexpr1if it can be converted tofalse; otherwise, returnsexpr2. Thus, when used with Boolean values,&&returnstrueif both operands aretrue; otherwise, returnsfalse.
In other words, Javascript does not coerce the operands to boolean values unless it has to.
4 && 5 Returns 5, not true.
In your case, if the first expression is undefined (which is convertible to false), then ctx will be false, and the second expression does not get evaluated. If the first expression is a function (which cannot be converted to false), then Javascript evaluates the second expression, and assigns it's value to the ctx variable.
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Yours and @Trott answers were correct. Although he did post first, I felt like this one gave a clearer explanation on what was actually happening. Not sure what the proper etiquette is.– Andy HinCommented Oct 14, 2012 at 2:05
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A minor nitpick here as well: the first expression would be
undefinedrather thannullifcanvasdidn't have a.getContextdefined. Commented Oct 14, 2012 at 2:07 -
9@AndyHin I'm way late here, but since you mentioned etiquette: You did the right thing. If you felt this was the best answer, then you should accept it, even if it wasn't the first correct answer. The point is to mark the answer that most helped you most and is most likely to help someone else coming to the site. The fact that I got my answer up first doesn't really factor into it. The first answer doesn't help someone needing help. The best answer does!– TrottCommented Jan 25, 2013 at 20:12
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Just curious, but what if we did want a boolean result? Instead of returning the result of the first truthy expression. Commented Aug 5, 2019 at 2:27
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You can use the Boolean object to coerce the result of the expression, i.e.,
Boolean(4 && 5) -> true,Boolean(undefined && 5) -> false– SethCommented Aug 5, 2019 at 17:56
It will assign the return value of canvas.getContext('2d') to ctx if canvas.getContext is actually a function.
The part on the left is just to avoid an error. It makes sure that canvas has a getContext property before trying to call getContext(). This way, the code won't call canvas.getContext() if the function doesn't exist. If it didn't check first and called when the function didn't exist, an error would be logged to the console and execution would halt.
