Reputation: 16438

Are these for loops different?

Here is the code

example 1

for (var k = 0, l = value.length; k < l; k++, index++) {
    // ...
}

example 2

var l = value.length;
for (var k = 0; k < l; k++, index++) {
    // ...
}

In example 1, variable l is defined inside of the 'for' brackets, so does it mean l is being defined every time the loop goes through? If so then example 2 would be more efficient right?

Upvotes: 0

Answers (5)

Dmitri Zaitsev

Reputation: 14046

I am surprised by the answers given here.

Example 1 is a well-known anti-pattern mentioned in many books and articles. It re-calculates l in every loop. Depending on how expensive this (or whatever other unnecessary) operation you put inside the for-call (except the 1st argument), the code may take much longer to execute.

Example 2 is better because the value of l is calculated only once and is cached. Caching is a very important concept and this simple examples demonstrates it. Also it will work even if you change value inside the loop, whereas the first Example could lead to unexpected results.

In my opinion, the only possible reason to use Example 1 is when you indeed need to recalculate l for every loop, e.g. when the loop changes value and you really want to update l.

Upvotes: -1