Most efficient way to generate a really long string (tens of megabytes) in JS

Question

I find myself needing to synthesize a ridiculously long string (like, tens of megabytes long) in JavaScript. (This is to slow down a CSS selector-matching operation to the point where it takes a measurable amount of time.)

The best way I've found to do this is

var really_long_string = (new Array(10*1024*1024)).join("x");

but I'm wondering if there's a more efficient way - one that doesn't involve creating a tens-of-megabytes array first.

Answer 1

Simply accumulating is vastly faster in Safari 5:

var x = "1234567890";
var iterations = 14;
for (var i = 0; i < iterations; i++) {
  x += x.concat(x);
}
alert(x.length); // 47829690

Essentially, you'll get x.length * 3^iterations characters.