JavaScript for loop syntax for array

Question

I have seen the following style of writing a for loop in JavaScript a number of times:

for(var i=0, n=myArray.length; i<n; i++) {
// do something
}

what is the advantage of that over the following?

for(var i=0; i<myArray.length; i++) {
// do something
}

possible duplicate of Is reading the `length` property of an array really that expensive an operation in JavaScript? and Do modern JavaScript JITers need array-length caching in loops? — Bergi
– Bergi, Commented Mar 30, 2014 at 14:48
@Bergi Yes, exact duplicate of both of those. Without knowing the answer it was impossible to search for those questions. What is the standard here. Should I delete this question? — Guy
– Guy, Commented Mar 31, 2014 at 17:48
Nah, if 5 people will vote for closing it will become tagged as a duplicate, that's enough. If you as the asker would, it should go pretty fast. — Bergi
– Bergi, Commented Mar 31, 2014 at 20:55

Quentin · Accepted Answer · 2014-03-30 14:44:57Z

3

In theory, it only has to look up the length of myArray once, instead of each time it goes around the loop.

In practise, these days I believe browsers have JS engines smart enough to optimise the performance impact of that look up away.

answered Mar 30, 2014 at 14:44

Quentin

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5 Comments

Shomz Over a year ago

For a price of additional variable.

Guy Over a year ago

But wouldn't reference the length member of an array be just as efficient as referencing n? i.e. they both need to be referenced in the same manner to extract the value.

Quentin Over a year ago

@Guy — No. "Get n" is less work than "Get myArray, now get the legnth of it".

Guy Over a year ago

Even if the JS engines wasn't optimized wouldn't the lookup time be the same?

Guy Over a year ago

Did you know that you can't accept an answer on stack overflow if the question is less than 10 minutes old? :)

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