JavaScript VM optimization on strings concatenation

Question

Some time ago declarations like the following were defined by poor performance:

var a = "my" +
"very" +
"very" +
"long" +
"string" +
"and" + 
"even" +
"longer";

I was told that every subsequent + operation causes an additional string to be created since they are immutable. At least, once that was an issue in Java programming language (mind StringBuffer vs. String).

I am talking about the more-or-less recent versions of the browsers of course.

The question is about JavaScript now: is it still not recommended or the runtime can squack (or should I say optimize) the issue like one above in milliseconds without any performance overhead?

You can do your own js performance tests on jsperf.com. For example jsperf.com/string-concat-js which seems to indicate that it doesn't matter with modern browsers. — James
– James, Commented Nov 16, 2011 at 14:01
@James: did it, looks I am correct (it took a very short time), but still curious for the global wisdom answer. — Alexander Abramovich
– Alexander Abramovich, Commented Nov 16, 2011 at 14:04

Šime Vidas · Accepted Answer · 2011-11-16 13:59:01Z

2

This makes more sense if you're concatenating a large amount of string values:

str = [
    'string1',
    'string2',
    'string3',
    'string4',
    'etc.'
].join( '' );

answered Nov 16, 2011 at 13:59

Šime Vidas

187k66 gold badges290 silver badges392 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Šime Vidas Over a year ago

@BreakPhreak What do you mean?

Alexander Abramovich Over a year ago

in the lines (like in my example) where the object is declared at its first time (and not mutated afterwards). am I clear enough, please?

Šime Vidas Over a year ago

@BreakPhreak That doesn't make a difference. This var x; x = ...; is equivalent to this var x = ...;.

Michael Borgwardt · Accepted Answer · 2011-11-16 14:10:16Z

A number of points:

What you cite was never a performance problem in Java; the compiler always optimized it using StringBuffer. Problems only arise when appending to a variable in a loop.
Java and JavaScript have absolutely nothing to do with each other.
The JavaScript runtime would have to work very hard to de-optimize the issue until it takes milliseconds. On a modern CPU, a millisecond is an eternity.
Premature optimization. Don't.
Profile/Benchmark before you optimize. Here's numbers from someone who did, but note that the results are 3 years old and thus meaningless.

Sergio Moura · Accepted Answer · 2011-11-16 14:01:20Z

0

I just found a simple implementation of the StringBuffer behaviour using JavaScript:

function StringBuffer() {
   this.buffer = [];
}

StringBuffer.prototype.append = function append(string) {
   this.buffer.push(string);
   return this;
 };

 StringBuffer.prototype.toString = function toString() {
   return this.buffer.join("");
 };

 var buf = new StringBuffer();

 buf.append("hello");
 buf.append("world");

 alert(buf.toString());

Source: http://social.msdn.microsoft.com/Forums/en-US/iewebdevelopment/thread/72429b15-73e2-4b4e-9288-9e1dfd47858b/

answered Nov 16, 2011 at 14:01

Sergio Moura

4,9321 gold badge25 silver badges39 bronze badges

1 Comment

Alexander Abramovich Over a year ago

Well, my question is more about the concatenation in declaration line. The optimisations there can be peanuts for the modern JS engines.

Collectives™ on Stack Overflow

JavaScript VM optimization on strings concatenation

3 Answers 3

3 Comments

Comments

1 Comment

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

Comments

1 Comment

Related