String concatenation vs string buffers in Javascript

Question

I was reading this book - Professional Javascript for Web Developers where the author mentions string concatenation is an expensive operation compared to using an array to s

Answer 1

In principle the book is right. Joining an array should be much faster than repeatedly concatenating to the same string. As a simple algorithm on immutable strings it is demonstrably faster.

The trick is: JavaScript authors, being largely non-expert dabblers, have written a load of code out there in the wild that uses concatenating, and relatively little ‘good’ code that using methods like array-join. The upshot is that browser authors can get a better improvement in speed on the average web page by catering for and optimising the ‘bad’, more common option of concatenation.

So that's what happened. The newer browser versions have some fairly hairy optimisation stuff that detects when you're doing a load of concatenations, and hacks it about so that internally it is working more like an array-join, at more or less the same speed.