Do common JavaScript implementations use string interning?

Question

Do common JavaScript engines, such as V8 and WebKit's JavaScriptCore, use string interning for JavaScript strings? Or do they actually keep multiple instances of identical strings in memory?

Answer 1

Yes. In general any literal string, identifier, or other constant string in JS source is interned. However implementation details (exactly what is interned for instance) varies, as well as when the interning occurs.

Note that a string value is not the same as a String Object though, String Objects are not interned because that would be fundamentally incorrect behaviour.

Answer 2

http://jsperf.com/strinterning

Yes in Chrome, no in Aurora 15 and FF 13! Comparing two strings is 85% slower than comparing two pointers in Firefox. However it's the same speed in Chrome, which is an indication that it is comparing two pointers.

Maybe the JS engine team at Mozilla should check their code...

Answer 3

Short answer: sometimes yes, sometimes no.

I also stumbled upon the same question and looked a bit into it. It seems that interning is done usually for string literals that are generated the same way (eg. always assigning the same string to a variable in the same loop), BUT I was also able to create an example which results in two identical strings being created with two different references:

As you can see, each string is stored twice, having different references.

This is the code I used to generate the duplicate strings:

const a = [];
const b = [];

for(let j  =1; j<= 100;++j){
    for(let i = 1; i <= 10000; ++i) a[i] = 'player 1 got 5 points from player 2' + i;
    for(let i = 1; i <= 10000; ++i) b[i] = 'player 1 got 5 points from player 2' + i;
}

It seems that string interning is done for string literals, but not for concatenated string values, but as you can see above, each concatenated string only appears twice, not 100x2 = 200 times, so there is still string interning done for concatenated strings created in the outer loop.