Fast sprintf() in Javascript

sprintf.js

/**
 * Note: this only currently supports the %s and %j formatters.
 *
 * The first call with any given formatter will be relatively slow, but every subsequent
 * call with the same formatter should be very fast (in V8).
 */

var cache = {};

var SINGLE_QUOTE = new RegExp('\'', 'g');

function populate(formatter) {
  var i, type;
  var key = formatter;
  var prev = 0;
  var arg = 1;
  var builder = 'return \'';

  formatter = formatter.replace(SINGLE_QUOTE, '\\\'');

  for (i = 0; i < formatter.length; i++) {
    if (formatter[i] === '%' && formatter[i - 1] !== '\\') {
      type = formatter[i + 1];

      switch (type) {
      case 's':
        builder += formatter.slice(prev, i) + '\' + arguments[' + arg + '] + \'';
        break;
      case 'j':
        builder += formatter.slice(prev, i) + '\' + JSON.stringify(arguments[' + arg + ']) + \'';
        break;
      }

      prev = i + 2;
      arg++;
    }
  }

  builder += formatter.slice(prev) + '\';';
  cache[key] = new Function(builder);
}


exports.sprintf = function(formatter) {
  if (!cache[formatter]) {
    populate(formatter);
  }

  return cache[formatter].apply(null, arguments);
};