¿Por qué ◎����◎ y ☺ no son nombres de variables JavaScript válidos?

Noté que en Internet Explorer (pero, desafortunadamente, no en los otros navegadores que probé), puedes usar algunos nombres de variables Unicode. Esto me alegró el día y estaba absolutamente encantado de poder escribir un código divertido cargado de Unicode como este:

var ктоείναι草泥马 = "You dirty horse.",
    happy☺n☺mat☺p☺eia = ":)Yay!",
    ಠ_ಠ = "emoticon";

alert(ктоείναι草泥马 + happy☺n☺mat☺p☺eia + ಠ_ಠ);

Sin embargo, por alguna razón ◎ܫ◎, ♨_♨y☺ son nombres de variables válidos.

¿Por qué hacer ಠ_ಠy 草泥马trabajar, pero no ◎ܫ◎, ♨_♨o ☺?

Sep 17 '11 07:09 Peter Olson

EcmaScript 262, sección 7.6 dice que los nombres deben comenzar con $, _ o una letra Unicode, y después de eso pueden contener esos caracteres o marcas combinadas Unicode, dígitos Unicode o puntuación del conector Unicode (y un par de caracteres de control de formato que son específicos del idioma.)

Por lo tanto, la diferencia entre identificadores permitidos y no permitidos en sus casos es probablemente si el carácter principal se considera una "letra".

Sep 17 '2011 00:09 broofa

Para citar nombres de variables JavaScript válidos , mi artículo resume las secciones de especificaciones relevantes:

Un identificador debe comenzar con $, _o cualquier carácter en las categorías Unicode “Letra mayúscula (Lu)” , “Letra minúscula (Ll)” , “Letra de título (Lt)” , “Letra modificadora (Lm)” , “Otra letra ( Lo)” o “Letra número (Nl)” .

El resto de la cadena puede contener los mismos caracteres, además de cualquier carácter no unido de ancho cero U+200C , carácter no unido de ancho cero U+200D y caracteres en las categorías Unicode “Marca sin espaciado (Mn)” , “Combinación de espaciado”. marca (Mc)” , “Número de dígito decimal (Nd)” o “Puntuación del conector (Pc)” .

También he creado una herramienta que le indicará si alguna cadena que ingrese es un nombre de variable JavaScript válido según ECMAScript 5.1 y Unicode 6.3:

PD: Si tuviera que resumir todas estas reglas en una única expresión regular solo ASCII para JavaScript , tendría más de 9.000 caracteres . Aquí lo tienes:

// ECMAScript 5.1 / Unicode v6.3.0:
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Feb 18 '2012 00:02 Mathias Bynens

Para las propiedades del objeto, puede utilizar la sintaxis de corchetes:

window["◎ܫ◎"] = true;
alert(window["◎ܫ◎"]); // alerts "true"

Apr 10 '2013 12:04 Tommi