node/tools/eslint/node_modules/es6-symbol/polyfill.js

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

'use strict';

var d              = require('d')
  , validateSymbol = require('./validate-symbol')

  , create = Object.create, defineProperties = Object.defineProperties
  , defineProperty = Object.defineProperty, objPrototype = Object.prototype
  , NativeSymbol, SymbolPolyfill, HiddenSymbol, globalSymbols = create(null)
  , isNativeSafe;

if (typeof Symbol === 'function') {
	NativeSymbol = Symbol;
	try {
		String(NativeSymbol());
		isNativeSafe = true;
	} catch (ignore) {}
}

var generateName = (function () {
	var created = create(null);
	return function (desc) {
		var postfix = 0, name, ie11BugWorkaround;
		while (created[desc + (postfix || '')]) ++postfix;
		desc += (postfix || '');
		created[desc] = true;
		name = '@@' + desc;
		defineProperty(objPrototype, name, d.gs(null, function (value) {
			// For IE11 issue see:
			// https://connect.microsoft.com/IE/feedbackdetail/view/1928508/
			//    ie11-broken-getters-on-dom-objects
			// https://github.com/medikoo/es6-symbol/issues/12
			if (ie11BugWorkaround) return;
			ie11BugWorkaround = true;
			defineProperty(this, name, d(value));
			ie11BugWorkaround = false;
		}));
		return name;
	};
}());

// 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('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('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))
	});
};
defineProperties(SymbolPolyfill, {
	for: d(function (key) {
		if (globalSymbols[key]) return globalSymbols[key];
		return (globalSymbols[key] = SymbolPolyfill(String(key)));
	}),
	keyFor: d(function (s) {
		var key;
		validateSymbol(s);
		for (key in globalSymbols) if (globalSymbols[key] === s) return key;
	}),

	// If there's native implementation of given symbol, let's fallback to it
	// to ensure proper interoperability with other native functions e.g. Array.from
	hasInstance: d('', (NativeSymbol && NativeSymbol.hasInstance) || SymbolPolyfill('hasInstance')),
	isConcatSpreadable: d('', (NativeSymbol && NativeSymbol.isConcatSpreadable) ||
		SymbolPolyfill('isConcatSpreadable')),
	iterator: d('', (NativeSymbol && NativeSymbol.iterator) || SymbolPolyfill('iterator')),
	match: d('', (NativeSymbol && NativeSymbol.match) || SymbolPolyfill('match')),
	replace: d('', (NativeSymbol && NativeSymbol.replace) || SymbolPolyfill('replace')),
	search: d('', (NativeSymbol && NativeSymbol.search) || SymbolPolyfill('search')),
	species: d('', (NativeSymbol && NativeSymbol.species) || SymbolPolyfill('species')),
	split: d('', (NativeSymbol && NativeSymbol.split) || SymbolPolyfill('split')),
	toPrimitive: d('', (NativeSymbol && NativeSymbol.toPrimitive) || SymbolPolyfill('toPrimitive')),
	toStringTag: d('', (NativeSymbol && NativeSymbol.toStringTag) || SymbolPolyfill('toStringTag')),
	unscopables: d('', (NativeSymbol && NativeSymbol.unscopables) || SymbolPolyfill('unscopables'))
});

// 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]));