JS: Fastest way to create all substrings starting at index 0

Question

So I'm looking for a way to generate all possible substrings from a larger String that start at index 0. So let's say we have

var a = "test";

Then I want to generate

"test", "tes", "te" and "t"

I imagine doing this with substring, substr or slice, and I tested them here: http://jsperf.com/loop-over-string

Now the slice method seems almost twice as fast as the other methods. Can anybody explain that? Or is there even faster ways to do this?

Answer 1

In your benchmark slice is faster because text's length decreases each iteration.

If you take a look at substr, substring and slice implementations in V8 you will realize that they use the same internal function %_SubString. They only manipulate its parameters at a negligible cost.

• String.prototype.slice: https://github.com/v8/v8/blob/master/src/string.js#L567
• String.prototype.substring: https://github.com/v8/v8/blob/master/src/string.js#L713
• String.prototype.substr: https://github.com/v8/v8/blob/master/src/string.js#L748