Way to test if a string is valid identifier name in JavaScript

Question

What's the most reliable way to test if a string can pass for correct variable name? I have couple of solutions on my mind, but I'm not quite sure which one to use:

// using <iframe> helper
// assuming I have hidden <iframe> inserted in document.body{}:
var _hidden_iframe_id = "iframe"+ Date.now();
var _fr = document.createElement("iframe");

_fr.setAttribute("id", _hidden_iframe_id);
_fr.setAttribute("name", _hidden_iframe_id);
_fr.style.display = "none";

document.body.appendChild(_fr);


// and .eval()-ing a string as a variable in <iframe>'s .contentWindow{}:

String.prototype.isidentifier = (function (owns, _frames, rnd, ifr_name) {

  // use private cache to save evaluated results
  var cache = {};

  // main
  return function () {

    // guard against code alike strings
    // takes everything up to left paren '(', inclusive
    var _varname = this.match(/^([^\(]*\(?)/)[1];
    var res, _win, _varval, _code;


    // been there?
    if (owns(cache, _varname)) 
      return cache[_varname];


    // flag
    res     = false;

    // random string to set as value 
    // of '_varname' in helper <iframe>
    _varval = ''+ rnd();


    try {


      // eval the iife in _win's context setting it's
      // return value to global `foo` variable. if it matches 
      // '_varvalue', the string can be used in `var` statement


      _code = 
        "foo = (function () {var "+ _varname + 
        " = '"+ _varval +"'; return "+ _varname + ";})();";

      _win = _frames[ifr_name];
      _win.eval(_code);


      // success?

      res = (_win.foo == _varval);

    } catch (e) {}


    // save the String 'varname' => Bool 'identifier-status'
    // mapping for future use

    return cache[_varname] = res;

  };

})(
  Function.prototype.call.bind(Object.prototype.hasOwnProperty), 
  window.frames, 
  Math.random, 
  _hidden_iframe_id
);


//
// ---
// or something like this, using RegExp:

String.prototype.isidentifier = function () {
  return (/^[a-z_\$][\w\$]*$/i).test(this);
};

// ?
//

I figure regex way would be the safest (and faster?) one, but actualy trying to initialize a variable and check if it has been set correctly would be more reliable (and insecure?) way to do it, right?

---

@SomeKittens Ux2666 Hey I've actually reproduced the regex you've posted using the 'ifame-version-function': runned it on each character in \u0000-\uffff range checking if it can be used as first character in variable name (yielded the exact set of characters as part of original regex where it checks the 1st character, ~48k characters), then did the same once again this time checking which characters can be used after 1st one (set first char to string a and looped through the character range appending each one to a and running the function on that...). Yielded ~50k characters, and more character points after ...\uff3f]*$/ original regex (supposed to check remaining part of variable string). Also expanded 'reserved-words-check' part according to ReservedWords@MDN docs.

Here's the modified regex: (9822 characters long)

/^(?!(?:abstract|boolean|break|byte|case|catch|char|class|const|continue|debugger|default|delete|do|double|else|enum|export|extends|final|finally|float|for|function|goto|if|implements|import|in|instanceof|int|interface|let|long|native|new|package|private|protected|public|return|short|static|super|switch|synchronized|this|throw|throws|transient|try|typeof|var|void|volatile|while|with|yield)$)[\x24\x41-\x5A\x5F\x61-\x7A\xAA\xB5\xBA\xC0-\xD6\xD8-\xF6\xF8-\u02C1\u02C6-\u02D1\u02E0-\u02E4\u02EC\u02EE\u0370-\u0374\u0376-\u0377\u037A-\u037D\u0386\u0388-\u038A\u038C\u038E-\u03A1\u03A3-\u03F5\u03F7-\u0481\u048A-\u0527\u0531-\u0556\u0559\u0561-\u0587\u05D0-\u05EA\u05F0-\u05F2\u0620-\u064A\u066E-\u066F\u0671-\u06D3\u06D5\u06E5-\u06E6\u06EE-\u06EF\u06FA-\u06FC\u06FF\u0710\u0712-\u072F\u074D-\u07A5\u07B1\u07CA-\u07EA\u07F4-\u07F5\u07FA\u0800-\u0815\u081A\u0824\u0828\u0840-\u0858\u08A0\u08A2-\u08AC\u0904-\u0939\u093D\u0950\u0958-\u0961\u0971-\u0977\u0979-\u097F\u0985-\u098C\u098F-\u0990\u0993-\u09A8\u09AA-\u09B0\u09B2\u09B6-\u09B9\u09BD\u09CE\u09DC-\u09DD\u09DF-\u09E1\u09F0-\u09F1\u0A05-\u0A0A\u0A0F-\u0A10\u0A13-\u0A28\u0A2A-\u0A30\u0A32-\u0A33\u0A35-\u0A36\u0A38-\u0A39\u0A59-\u0A5C\u0A5E\u0A72-\u0A74\u0A85-\u0A8D\u0A8F-\u0A91\u0A93-\u0AA8\u0AAA-\u0AB0\u0AB2-\u0AB3\u0AB5-\u0AB9\u0ABD\u0AD0\u0AE0-\u0AE1\u0B05-\u0B0C\u0B0F-\u0B10\u0B13-\u0B28\u0B2A-\u0B30\u0B32-\u0B33\u0B35-\u0B39\u0B3D\u0B5C-\u0B5D\u0B5F-\u0B61\u0B71\u0B83\u0B85-\u0B8A\u0B8E-\u0B90\u0B92-\u0B95\u0B99-\u0B9A\u0B9C\u0B9E-\u0B9F\u0BA3-\u0BA4\u0BA8-\u0BAA\u0BAE-\u0BB9\u0BD0\u0C05-\u0C0C\u0C0E-\u0C10\u0C12-\u0C28\u0C2A-\u0C33\u0C35-\u0C39\u0C3D\u0C58-\u0C59\u0C60-\u0C61\u0C85-\u0C8C\u0C8E-\u0C90\u0C92-\u0CA8\u0CAA-\u0CB3\u0CB5-\u0CB9\u0CBD\u0CDE\u0CE0-\u0CE1\u0CF1-\u0CF2\u0D05-\u0D0C\u0D0E-\u0D10\u0D12-\u0D3A\u0D3D\u0D4E\u0D60-\u0D61\u0D7A-\u0D7F\u0D85-\u0D96\u0D9A-\u0DB1\u0DB3-\u0DBB\u0DBD\u0DC0-\u0DC6\u0E01-\u0E30\u0E32-\u0E33\u0E40-\u0E46\u0E81-\u0E82\u0E84\u0E87-\u0E88\u0E8A\u0E8D\u0E94-\u0E97\u0E99-\u0E9F\u0EA1-\u0EA3\u0EA5\u0EA7\u0EAA-\u0EAB\u0EAD-\u0EB0\u0EB2-\u0EB3\u0EBD\u0EC0-\u0EC4\u0EC6\u0EDC-\u0EDF\u0F00\u0F40-\u0F47\u0F49-\u0F6C\u0F88-\u0F8C\u1000-\u102A\u103F\u1050-\u1055\u105A-\u105D\u1061\u1065-\u1066\u106E-\u1070\u1075-\u1081\u108E\u10A0-\u10C5\u10C7\u10CD\u10D0-\u10FA\u10FC-\u1248\u124A-\u124D\u1250-\u1256\u1258\u125A-\u125D\u1260-\u1288\u128A-\u128D\u1290-\u12B0\u12B2-\u12B5\u12B8-\u12BE\u12C0\u12C2-\u12C5\u12C8-\u12D6\u12D8-\u1310\u1312-\u1315\u1318-\u135A\u1380-\u138F\u13A0-\u13F4\u1401-\u166C\u166F-\u167F\u1681-\u169A\u16A0-\u16EA\u16EE-\u16F0\u1700-\u170C\u170E-\u1711\u1720-\u1731\u1740-\u1751\u1760-\u176C\u176E-\u1770\u1780-\u17B3\u17D7\u17DC\u1820-\u1877\u1880-\u18A8\u18AA\u18B0-\u18F5\u1900-\u191C\u1950-\u196D\u1970-\u1974\u1980-\u19AB\u19C1-\u19C7\u1A00-\u1A16\u1A20-\u1A54\u1AA7\u1B05-\u1B33\u1B45-\u1B4B\u1B83-\u1BA0\u1BAE-\u1BAF\u1BBA-\u1BE5\u1C00-\u1C23\u1C4D-\u1C4F\u1C5A-\u1C7D\u1CE9-\u1CEC\u1CEE-\u1CF1\u1CF5-\u1CF6\u1D00-\u1DBF\u1E00-\u1F15\u1F18-\u1F1D\u1F20-\u1F45\u1F48-\u1F4D\u1F50-\u1F57\u1F59\u1F5B\u1F5D\u1F5F-\u1F7D\u1F80-\u1FB4\u1FB6-\u1FBC\u1FBE\u1FC2-\u1FC4\u1FC6-\u1FCC\u1FD0-\u1FD3\u1FD6-\u1FDB\u1FE0-\u1FEC\u1FF2-\u1FF4\u1FF6-\u1FFC\u2071\u207F\u2090-\u209C\u2102\u2107\u210A-\u2113\u2115\u2119-\u211D\u2124\u2126\u2128\u212A-\u212D\u212F-\u2139\u213C-\u213F\u2145-\u2149\u214E\u2160-\u2188\u2C00-\u2C2E\u2C30-\u2C5E\u2C60-\u2CE4\u2CEB-\u2CEE\u2CF2-\u2CF3\u2D00-\u2D25\u2D27\u2D2D\u2D30-\u2D67\u2D6F\u2D80-\u2D96\u2DA0-\u2DA6\u2DA8-\u2DAE\u2DB0-\u2DB6\u2DB8-\u2DBE\u2DC0-\u2DC6\u2DC8-\u2DCE\u2DD0-\u2DD6\u2DD8-\u2DDE\u2E2F\u3005-\u3007\u3021-\u3029\u3031-\u3035\u3038-\u303C\u3041-\u3096\u309D-\u309F\u30A1-\u30FA\u30FC-\u30FF\u3105-\u312D\u3131-\u318E\u31A0-\u31BA\u31F0-\u31FF\u3400-\u4DB5\u4E00-\u9FCC\uA000-\uA48C\uA4D0-\uA4FD\uA500-\uA60C\uA610-\uA61F\uA62A-\uA62B\uA640-\uA66E\uA67F-\uA697\uA6A0-\uA6EF\uA717-\uA71F\uA722-\uA788\uA78B-\uA78E\uA790-\uA793\uA7A0-\uA7AA\uA7F8-\uA801\uA803-\uA805\uA807-\uA80A\uA80C-\uA822\uA840-\uA873\uA882-\uA8B3\uA8F2-\uA8F7\uA8FB\uA90A-\uA925\uA930-\uA946\uA960-\uA97C\uA984-\uA9B2\uA9CF\uAA00-\uAA28\uAA40-\uAA42\uAA44-\uAA4B\uAA60-\uAA76\uAA7A\uAA80-\uAAAF\uAAB1\uAAB5-\uAAB6\uAAB9-\uAABD\uAAC0\uAAC2\uAADB-\uAADD\uAAE0-\uAAEA\uAAF2-\uAAF4\uAB01-\uAB06\uAB09-\uAB0E\uAB11-\uAB16\uAB20-\uAB26\uAB28-\uAB2E\uABC0-\uABE2\uAC00-\uD7A3\uD7B0-\uD7C6\uD7CB-\uD7FB\uF900-\uFA6D\uFA70-\uFAD9\uFB00-\uFB06\uFB13-\uFB17\uFB1D\uFB1F-\uFB28\uFB2A-\uFB36\uFB38-\uFB3C\uFB3E\uFB40-\uFB41\uFB43-\uFB44\uFB46-\uFBB1\uFBD3-\uFD3D\uFD50-\uFD8F\uFD92-\uFDC7\uFDF0-\uFDFB\uFE70-\uFE74\uFE76-\uFEFC\uFF21-\uFF3A\uFF41-\uFF5A\uFF66-\uFFBE\uFFC2-\uFFC7\uFFCA-\uFFCF\uFFD2-\uFFD7\uFFDA-\uFFDC][\x24\x30-\x39\x41-\x5A\x5F\x61-\x7A\xA0\xAA\xB5\xBA\xC0-\xD6\xD8-\xF6\xF8-\u02C1\u02C6-\u02D1\u02E0-\u02E4\u02EC\u02EE\u0300-\u0374\u0376-\u0377\u037A-\u037D\u0386\u0388-\u038A\u038C\u038E-\u03A1\u03A3-\u03F5\u03F7-\u0481\u0483-\u0487\u048A-\u0527\u0531-\u0556\u0559\u0561-\u0587\u0591-\u05BD\u05BF\u05C1-\u05C2\u05C4-\u05C5\u05C7\u05D0-\u05EA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//

Answer 1

In one 'simple' regex:

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Yeah, not quite. The problem arises from the fact that so many characters are perfectly legal:

An identifier must start with $, _, or any character in the Unicode categories “Uppercase letter (Lu)”, “Lowercase letter (Ll)”, “Titlecase letter (Lt)”, “Modifier letter (Lm)”, “Other letter (Lo)”, or “Letter number (Nl)”.

The rest of the string can contain the same characters, plus any U+200C zero width non-joiner characters, U+200D zero width joiner characters, and characters in the Unicode categories “Non-spacing mark (Mn)”, “Spacing combining mark (Mc)”, “Decimal digit number (Nd)”, or “Connector punctuation (Pc)”.

For more information, check this answer, which contains the above and more.