Question

I have a code like:

$.each($.parseJSON(data).content,function(){});

I would like to know if the $.parseJSON(data).content will be evaluated on each iteration, or just once? This will affect the performance significantly.

Answer 1

The arguments of a function call are always evaluated before the function is called. This isn’t jQuery specific. See the specification:

3 . Let argList be the result of evaluating Arguments, producing an internal list of argument values (see 11.2.4).

…

8 . Return the result of calling the [[Call]] internal method on func, providing thisValue as the this value and providing the list argList as the argument values.

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For clarity, this is not the case in for loops:

for(var i = 0; i < $.parseJSON(data).content.length; i++) {

}

Here, the second expression is evaluated every time. This makes sense, since in a simple loop you want to reevaluate the condition i < length at every iteration. But you don’t want to run $.parseJSON at every iteration. So in for loops, it does make sense to factor out the expression.