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, Set (requires ES5 property descriptor support)
• Promise
• String:
  • fromCodePoint() (a standalone shim is also available)
  • raw()
• 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()
  • isSafeInteger()
  • isFinite()
  • parseInt()
  • parseFloat()
• Array:
  • from() (a standalone shim is also available)
  • of() (a standalone shim is also available)
• Array.prototype:
  • copyWithin()
  • 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()
  • asinh()
  • atanh()
  • cbrt()
  • clz32()
  • cosh()
  • expm1()
  • fround()
  • hypot()
  • imul()
  • log10()
  • log1p()
  • log2()
  • sign()
  • sinh()
  • tanh()
  • trunc()

Math functions’ accuracy is 1e-11.

• Reflect

  • apply()
  • construct()
  • defineProperty()
  • deleteProperty()
  • get()
  • getOwnPropertyDescriptor()
  • getPrototypeOf()
  • 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
  • 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.

License

The project was initially based on es6-shim by Axel Rauschmayer.