es6-symbol/polyfill.js

// ES2015 Symbol polyfill for environments that do not (or partially) support it

"use strict";

var d                    = require("d")
  , validateSymbol       = require("./validate-symbol")
  , NativeSymbol         = require("ext/global-this").Symbol
  , generateName         = require("./lib/private/generate-name")
  , setupStandardSymbols = require("./lib/private/setup/standard-symbols")
  , setupSymbolRegistry  = require("./lib/private/setup/symbol-registry");

var create = Object.create
  , defineProperties = Object.defineProperties
  , defineProperty = Object.defineProperty;

var SymbolPolyfill, HiddenSymbol, isNativeSafe;

if (typeof NativeSymbol === "function") {
	try {
		String(NativeSymbol());
		isNativeSafe = true;
	} catch (ignore) {}
} else {
	NativeSymbol = null;
}

// Internal constructor (not one exposed) for creating Symbol instances.
// This one is used to ensure that `someSymbol instanceof Symbol` always return false
HiddenSymbol = function Symbol(description) {
	if (this instanceof HiddenSymbol) throw new TypeError("Symbol is not a constructor");
	return SymbolPolyfill(description);
};

// Exposed `Symbol` constructor
// (returns instances of HiddenSymbol)
module.exports = SymbolPolyfill = function Symbol(description) {
	var symbol;
	if (this instanceof Symbol) throw new TypeError("Symbol is not a constructor");
	if (isNativeSafe) return NativeSymbol(description);
	symbol = create(HiddenSymbol.prototype);
	description = description === undefined ? "" : String(description);
	return defineProperties(symbol, {
		__description__: d("", description),
		__name__: d("", generateName(description))
	});
};

setupStandardSymbols(SymbolPolyfill);
setupSymbolRegistry(SymbolPolyfill);

// Internal tweaks for real symbol producer
defineProperties(HiddenSymbol.prototype, {
	constructor: d(SymbolPolyfill),
	toString: d("", function () { return this.__name__; })
});

// Proper implementation of methods exposed on Symbol.prototype
// They won't be accessible on produced symbol instances as they derive from HiddenSymbol.prototype
defineProperties(SymbolPolyfill.prototype, {
	toString: d(function () { return "Symbol (" + validateSymbol(this).__description__ + ")"; }),
	valueOf: d(function () { return validateSymbol(this); })
});
defineProperty(
	SymbolPolyfill.prototype, SymbolPolyfill.toPrimitive,
	d("", function () {
		var symbol = validateSymbol(this);
		if (typeof symbol === "symbol") return symbol;
		return symbol.toString();
	})
);
defineProperty(SymbolPolyfill.prototype, SymbolPolyfill.toStringTag, d("c", "Symbol"));

// Proper implementaton of toPrimitive and toStringTag for returned symbol instances
defineProperty(
	HiddenSymbol.prototype, SymbolPolyfill.toStringTag,
	d("c", SymbolPolyfill.prototype[SymbolPolyfill.toStringTag])
);

// Note: It's important to define `toPrimitive` as last one, as some implementations
// implement `toPrimitive` natively without implementing `toStringTag` (or other specified symbols)
// And that may invoke error in definition flow:
// See: https://github.com/medikoo/es6-symbol/issues/13#issuecomment-164146149
defineProperty(
	HiddenSymbol.prototype, SymbolPolyfill.toPrimitive,
	d("c", SymbolPolyfill.prototype[SymbolPolyfill.toPrimitive])
);