es-map/lib/support.js

An ES-spec-compliant Map shim/polyfill/replacement that works as far down as ES3

'use strict';

var valueOrFalseIfThrows = function (cb) {
	return function () {
		try {
			return cb();
		} catch (_) {
			return false;
		}
	};
};

// Safari 8, for example, doesn't accept an iterable.
exports.mapAcceptsArguments = valueOrFalseIfThrows(function () {
	return new Map([[1, 2]]).get(1) === 2;
});

exports.mapUsesSameValueZero = function () {
	// Chrome 38-42, node 0.11/0.12, iojs 1/2 also have a bug when the Map has a size > 4
	var m = new Map([
		// eslint-disable-next-line no-magic-numbers
		[1, 0], [2, 0], [3, 0], [4, 0]
	]);
	m.set(-0, m);
	return m.get(0) === m && m.get(-0) === m && m.has(0) && m.has(-0);
};

exports.mapSupportsChaining = function () {
	var testMap = new Map();
	return testMap.set(1, 2) === testMap;
};

var mapSupportsSubclassing = valueOrFalseIfThrows(function () {
	// without Object.setPrototypeOf, subclassing is not possible anyway
	if (!Object.setPrototypeOf) {
		return true;
	}

	var Sub = function Subclass(arg) {
		var o = new Map(arg);
		Object.setPrototypeOf(o, Subclass.prototype);
		return o;
	};
	Object.setPrototypeOf(Sub, Map);
	Object.setPrototypeOf(Sub.prototype, Map.prototype);

	var m = new Sub([]);
	/*
	 * Firefox 32 is ok with the instantiating the subclass but will
	 * throw when the map is used.
	 */
	m.set(1, 1);
	return m instanceof Sub;
});

// In Firefox 25 at least, Map and Set are callable without "new"
var mapRequiresNew = function () {
	try {
		// eslint-disable-next-line new-cap
		return !(Map() instanceof Map);
	} catch (e) {
		return e instanceof TypeError;
	}
};

exports.mapCompliantConstructor = function () {
	return Map.length === 0 && mapSupportsSubclassing() && mapRequiresNew();
};

/*
 * In Firefox 25, the iterator throws when it finishes
 * In Safari 8, new Map().keys().next is not a function
 */
exports.mapIterationFinishes = valueOrFalseIfThrows(function () {
	return new Map().keys().next().done;
});

exports.mapHasOldFirefoxInterface = function () {
	/*
	 * In Firefox < 19, Map#clear does not exist.
	 * In Firefox < 23, Map#size is a function.
	 * In Firefox < 24, Set#entries/keys/values do not exist: https://bugzilla.mozilla.org/show_bug.cgi?id=869996
	 * In Firefox 24, Map and Set do not implement forEach
	 */
	return typeof Map.prototype.clear !== 'function'
		|| new Map().size !== 0
		|| typeof Map.prototype.keys !== 'function'
		|| typeof Map.prototype.forEach !== 'function';
};