javascript引擎是否执行多维数组的任何优化？

Question

I know that modern javascript engines create hidden classes behind the scenes, so it's not recommended to change objects at runtime (if you have a large set of objects with same structure).

Has anyone heard about internal optimizations of two-dimentional arrays?

I create a two-dimentional array this way:

var map = [];
for (i = 0; i < 100000; i++) {
    map.push(Array(2));
} 

I'm planning to break the nice structure like this:

map[100].push(/* third element! */)

Will this have a dramatic impact on performance?

Answer 1

V8 doesn't do any optimizations for multi-dimensional arrays, so there should be no measurable impact on performance there.

I can't speak for other JavaScript engines, but would find it surprising if the pattern you describe had negative impact anywhere. The kind of optimization you're talking about typically requires making assumptions and guarding them with checks. Given the highly dynamic nature of JavaScript arrays, this is brittle enough for one-dimensional arrays; checking any non-trivial property of two-dimensional arrays is probably just not worth the cost.