utf8ToBytes

function utf8ToBytes (str) {
  // TODO(user): Use native implementations if/when available
  var p = 0
  var buf = new ArrayBuffer(str.length * 1.5) // fixme Use better algo?
  var out = new Uint8Array(buf)
  for (var i = 0; i < str.length; i++) {
    var c = str.charCodeAt(i)
    if (c < 128) {
      out[p++] = c
    } else if (c < 2048) {
      out[p++] = (c >> 6) | 192
      out[p++] = (c & 63) | 128
    } else if (
      ((c & 0xFC00) == 0xD800) && (i + 1) < str.length &&
      ((str.charCodeAt(i + 1) & 0xFC00) == 0xDC00)) {
      // Surrogate Pair
      c = 0x10000 + ((c & 0x03FF) << 10) + (str.charCodeAt(++i) & 0x03FF)
      out[p++] = (c >> 18) | 240
      out[p++] = ((c >> 12) & 63) | 128
      out[p++] = ((c >> 6) & 63) | 128
      out[p++] = (c & 63) | 128
    } else {
      out[p++] = (c >> 12) | 224
      out[p++] = ((c >> 6) & 63) | 128
      out[p++] = (c & 63) | 128
    }
  }
  return out.slice(0, p)
}

Much fastest way is actually to use TextEncoder and TextDecoder apis:
https://stackoverflow.com/questions/6965107/converting-between-strings-and-arraybuffers