Store list.length in a variable

[p1pchenk0]

What problem does this feature solve?

Increase performance during Vue preparations. During Vue initialization process there is a function makeMap, which splits string of items into array and then create Map-like object. But during for-loop every iteration it checks list.length value, which can be stored in separate variable before running for-loop. For arrays of 40 this can improve performance

What does the proposed API look like?

    function makeMap (str, expectsLowerCase) {
        var map = Object.create(null);
        var list = str.split(',');
        var listLength = list.length;  // <--  here it is

        for (var i = 0; i < listLength; i++) {
            map[list[i]] = true;
        }
        
        return expectsLowerCase ? function (val) { return map[val.toLowerCase()] } : function (val) { return map[val] }
  }

[p1pchenk0]

during exploring Vue source code I've found other places, too, where this length caching can be applied. I can create pull-request

[Justineo]

For code refactoring regarding to performance, we need a benchmark to show the changes are worthwhile. And according to some existing benchmarks I found the results are almost identical (sometimes the uncached version is even faster) on latest browsers.

[p1pchenk0]

@Justineo, I tried running your code on Chrome 69 and got

Anyway, I'll create appropriate benchmark tests and add links here

[Justineo]

If you run the benchmark above multiple times you’ll see there is no clear winner here (at least on modern browsers).

[yyx990803]

Thanks for the suggestion - however, in general modern JavaScript engines are smart enough to notice static array lengths and in practice this does not result in noticeable differences - especially for cases like makeMap which is insignificant in terms of total startup cost.