Do common JavaScript implementations use string interning?

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?

Answers:

Answer

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

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 primitives 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:

enter image description here

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

Answer

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...

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