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?

Answer 1

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.