把超出 ascii 范围的字符（如中文）等转为基于 ascii 范围字符的转义表达形式（https://github.com/mishoo/UglifyJS2/blob/master/lib/output.js#L76）

to_ascii.js

// from https://github.com/mishoo/UglifyJS2/blob/harmony/lib/output.js
function to_ascii(str) {
  return str.replace(/[\ud800-\udbff][\udc00-\udfff]|[\u0000-\u001f\u007f-\uffff]/g, function (ch) {
    var code = get_full_char_code(ch, 0).toString(16);

    if (code.length <= 2) {
      while (code.length < 2) code = "0" + code;
      return "\\x" + code;
    } else {
      while (code.length < 4) code = "0" + code;
      return "\\u" + code;
    }
  });
};

function get_full_char_code(str, pos) {
  // https://en.wikipedia.org/wiki/Universal_Character_Set_characters#Surrogates
  if (is_surrogate_pair_head(str.charAt(pos))) {
    return 0x10000 + (str.charCodeAt(pos) - 0xd800 << 10) + str.charCodeAt(pos + 1) - 0xdc00;
  }
  return str.charCodeAt(pos);
}

function is_surrogate_pair_head(code) {
  if (typeof code === "string")
    code = code.charCodeAt(0);

  return code >= 0xd800 && code <= 0xdbff;
}