ES6 Shim

Provides compatibility shims so that legacy JavaScript engines behave as closely as possible to ECMAScript 6 (Harmony).

HTML version of the final ECMAScript 6 spec

Installation

If you want to use it in browser:

Just include es6-shim before your scripts.
Include es5-shim especially if your browser doesn't support ECMAScript 5 - but every JS engine requires the es5-shim to correct broken implementations, so it's strongly recommended to always include it. Additionally, es5-shim should be loaded before es6-shim.

For node.js, io.js, or any npm-managed workflow (this is the recommended method):

npm install es6-shim

Alternative methods:

component install paulmillr/es6-shim if you’re using component(1).
bower install es6-shim if you’re using Bower.

In both browser and node you may also want to include unorm; see the String.prototype.normalize section for details.

Safe shims

Map (requires ES5 property descriptor support) (a standalone shim is also available)
Set (requires ES5 property descriptor support) (a standalone shim is also available)
Promise
String:
- fromCodePoint() (a standalone shim is also available)
- raw() (a stanadlone shim is also available)
String.prototype:
- codePointAt() (a standalone shim is also available)
- endsWith() (a standalone shim is also available)
- includes() (a standalone shim is also available)
- repeat() (a standalone shim is also available)
- startsWith() (a standalone shim is also available)
RegExp:
- new RegExp, when given a RegExp as the pattern, will no longer throw when given a "flags" string argument. (requires ES5)
RegExp.prototype:
- flags (requires ES5) (a standalone shim is also available)
- [Symbol.match] (requires native Symbols)
- [Symbol.replace] (requires native Symbols)
- [Symbol.search] (requires native Symbols)
- [Symbol.split] (requires native Symbols)
- toString
Number:
- binary and octal literals: Number('0b1') and Number('0o7')
- EPSILON
- MAX_SAFE_INTEGER
- MIN_SAFE_INTEGER
- isNaN() (a standalone shim is also available)
- isInteger() ([a standalone shim is also available(https://www.npmjs.com/package/number.isinteger))
- isSafeInteger()(a standalone shim is also available)
- isFinite() (a standalone shim is also available)
- parseInt() (a standalone shim is also available)
- parseFloat()
Array:
- from() (a standalone shim is also available)
- of() (a standalone shim is also available)
Array.prototype:
- copyWithin() (a standalone shim is also available)
- entries() (a standalone shim is also available)
- fill()
- find() (a standalone shim is also available)
- findIndex() (a standalone shim is also available)
- keys() (a standalone shim is also available)
- values() (a standalone shim is also available)
- indexOf() (ES6 errata) (a standalone shim is also available)
Object:
- assign() (a standalone shim is also available)
- is() (a standalone shim is also available)
- keys() (in ES5, but no longer throws on non-object non-null/undefined values in ES6) (a standalone shim is also available
- setPrototypeOf() (IE >= 11)
Function.prototype:
- name (es6-sham, covers IE 9-11) (a standalone shim is also available
Math:
- acosh() (a standalone shim is also available)
- asinh()
- atanh() (a standalone shim is also available)
- cbrt() (a standalone shim is also available)
- clz32() (a standalone shim is also available)
- cosh()
- expm1()
- fround() (a standalone shim is also available)
- hypot()
- imul() (a standalone shim is also available)
- log10() (a standalone shim is also available)
- log1p() (a standalone shim is also available)
- log2()
- sign() (a standalone shim is also available)
- sinh()
- tanh()
- trunc()

Math functions’ accuracy is 1e-11.

Reflect
- apply() (a standalone shim is also available)
- construct()
- defineProperty()
- deleteProperty()
- get()
- getOwnPropertyDescriptor()
- getPrototypeOf() (a standalone shim is also available)
- has()
- isExtensible()
- ownKeys() (a standalone shim is also available)
- preventExtensions()
- set()
- setPrototypeOf()
Symbol (only if it already exists)
- match (and corresponding String#match, String#startsWith, String#endsWith, String#includes, RegExp support)
- replace (and corresponding String#replace support)
- search (and corresponding String#search support)
- split (and corresponding String#split support)

Well-known symbols will only be provided if the engine already has Symbol support.

String.prototype Annex B HTML methods (a standalone shim is also available)
- anchor()
- big()
- blink()
- bold()
- fixed()
- fontcolor()
- fontsize()
- italics()
- link()
- small()
- strike()
- sub()
- sup()

These methods are part of "Annex B", which means that although they are a defacto standard, you shouldn't use them. None the less, the es6-shim provides them and normalizes their behavior across browsers.

Subclassing

The Map, Set, and Promise implementations are subclassable. You should use the following pattern to create a subclass in ES5 which will continue to work in ES6:

require('es6-shim');

function MyPromise(exec) {
  var promise = new Promise(exec);
  Object.setPrototypeOf(promise, MyPromise.prototype);
  // ...
  return promise;
}
Object.setPrototypeOf(MyPromise, Promise);
MyPromise.prototype = Object.create(Promise.prototype, {
  constructor: { value: MyPromise }
});

String.prototype.normalize

Including a proper shim for String.prototype.normalize would increase the size of this library by a factor of more than 4. So instead we recommend that you install the unorm package alongside es6-shim if you need String.prototype.normalize. See https://github.com/paulmillr/es6-shim/issues/134 for more discussion.

WeakMap shim

It is not possible to implement WeakMap in pure javascript. The es6-collections implementation doesn't hold values strongly, which is critical for the collection. es6-shim decided to not include an incorrect shim.

WeakMap has very unusual use-cases, so you probably won't need it at all (use simple Map instead).

Getting started

require('es6-shim');
var assert = require('assert');

assert.equal(true, 'abc'.startsWith('a'));
assert.equal(false, 'abc'.endsWith('a'));
assert.equal(true, 'john alice'.includes('john'));
assert.equal('123'.repeat(2), '123123');

assert.equal(false, NaN === NaN);
assert.equal(true, Object.is(NaN, NaN));
assert.equal(true, -0 === 0);
assert.equal(false, Object.is(-0, 0));

var result = Object.assign({ a: 1 }, { b: 2 });
assert.deepEqual(result, { a: 1, b: 2 });

assert.equal(true, isNaN('a'));
assert.equal(false, Number.isNaN('a'));
assert.equal(true, Number.isNaN(NaN));

assert.equal(true, isFinite('123'));
assert.equal(false, Number.isFinite('123'));
assert.equal(false, Number.isFinite(Infinity));

// Tests if value is a number, finite,
// >= -9007199254740992 && <= 9007199254740992 and floor(value) === value
assert.equal(false, Number.isInteger(2.4));

assert.equal(1, Math.sign(400));
assert.equal(0, Math.sign(0));
assert.equal(-1, Math.sign(-400));

var found = [5, 10, 15, 10].find(function (item) { return item / 2 === 5; });
assert.equal(10, found);

var foundIndex = [5, 10, 15, 10].findIndex(function (item) { return item / 2 === 5; });
assert.equal(1, foundIndex);

// Replacement for `{}` key-value storage.
// Keys can be anything.
var map = new Map([['Bob', 42], ['Foo', 'bar']]);
map.set('John', 25);
map.set('Alice', 400);
map.set(['meh'], 555);
assert.equal(undefined, map.get(['meh'])); // undefined because you need to use exactly the same object.
map.delete('Alice');
map.keys();
map.values();
assert.equal(4, map.size);

// Useful for storing unique items.
var set = new Set([0, 1]);
set.add(2);
set.add(5);
assert.equal(true, set.has(0));
assert.equal(true, set.has(1));
assert.equal(true, set.has(2));
assert.equal(false, set.has(4));
assert.equal(true, set.has(5));
set.delete(5);
assert.equal(false, set.has(5));

// Promises, see
// http://www.slideshare.net/domenicdenicola/callbacks-promises-and-coroutines-oh-my-the-evolution-of-asynchronicity-in-javascript
// https://github.com/petkaantonov/bluebird/#what-are-promises-and-why-should-i-use-them
Promise.resolve(5).then(function (value) {
  assert.equal(value, 5);
  if (value) throw new Error('whoops!');
  // do some stuff
  return anotherPromise();
}).catch(function (e) {
  assert.equal(e.message, 'whoops!');
  assert.equal(true, e instanceof Error);
  // any errors thrown asynchronously end up here
});

Caveats

Object.setPrototypeOf / Reflect.setPrototypeOf
- Note that null objects (Object.create(null), eg, an object with null as its [[Prototype]]) can not have their [[Prototype]] changed except via a native Object.setPrototypeOf.
Well-known Symbols
- In order to make them work cross-realm, these are created with the global Symbol registry via Symbol.for. This does not violate the spec, but it does mean that Symbol.for('Symbol.search') === Symbol.search will be true, which it would not by default in a fresh compliant realm.