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

I find myself needing to synthesize a ridiculously long string (like, tens of megabytes long) in JavaScript. (This is开发者_StackOverflow社区 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.

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For ES6:

'x'.repeat(10*1024*1024)

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The previously accepted version uses String.prototype.concat() which is vastly slower than using the optimized string concatenating operator, +. MDN also recommends to keep away from using it in speed critical code.

I have made three versions of the above code to show the speed differences in a JsPerf. Converting it to using only using concat is only a third as fast as only using the string concatenating operator (Chrome - your mileage will vary). The edited version below will run twice as fast in Chrome

var x = '1234567890'
var iterations = 14
for (var i = 0; i < iterations; i++) {
  x += x + x
}

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This is the more efficient algorithm for generating very long strings in javascript:

function stringRepeat(str, num) {
    num = Number(num);

    var result = '';
    while (true) {
        if (num & 1) { // (1)
            result += str;
        }
        num >>>= 1; // (2)
        if (num <= 0) break;
        str += str;
    }

    return result;
}

more info here: http://www.2ality.com/2014/01/efficient-string-repeat.html.

Alternatively, in ECMA6 you can use String.prototype.repeat() method.

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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.

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Not sure if this is a great implementation, but here's a general function based on @oligofren's solution:

function repeat(ch, len) {
  var result = ch;
  var halfLength = len / 2;
  while (result.length < len) {
    if (result.length <= halfLength) {
      result += result;
    } else {
      return result + repeat(ch, len - result.length);
    }
  }

  return result;
}

This assumes that concatenating a large string is faster than a series of small strings.