CODEWITHSUNDEEP
javascriptprofiling
Álvaro Mondéjar
Álvaro Mondéjar

Reputation: 484

Why `var a = "..."` is faster than `var a = new String("...")`?

I'm trying to understand how Javascript works. Profiling some code, I've found that declarations in a "typed" oriented way are slower than declarations that not are "typed". Of course, I know that JS is not a typed language.

I've tested this on Firefox, Chrome and Opera, always with same results.

var repetitions = 10000000;

console.time("a");
for (var i=0; i<repetitions; i++) {
    var a = "...";
}
console.timeEnd("a");

console.time("b");
for (var i=0; i<repetitions; i++) {
    var b = new String("...");
}
console.timeEnd("b")

Upvotes: 0

Views: 60

Answers (1)

georg
georg

Reputation: 214949

In V8 specifically, if you inspect the generated bytecode (node --print-bytecode), you'll get something like this:

> return "..."

LdaConstant [0]
Return 

> return new String("...")

LdaGlobal [0], [0]
Star r0
LdaConstant [1]
Star r1
Ldar r0
Construct r0, r1-r1, [2]
Return

which basically means that a string literal just loads the pointer from the constants pool, while new String involves far more steps - load the global String object, load the constant, invoke the generic Construct method. I guess things are not much different in FF or any other engine.

Upvotes: 1

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