es6-shim

What is es6-shim?

The es6-shim package is designed to provide compatibility shims so that legacy JavaScript engines behave as closely as possible to ECMAScript 6 (ES6). It includes shims for new methods and functions introduced in ES6, allowing developers to use these features in environments that do not yet support them natively.

What are es6-shim's main functionalities?

Array.prototype methods

Adds new methods to Array.prototype like find, findIndex, etc., enabling more expressive operations on arrays.

[1, 2, 3].find(x => x == 2)

String.prototype methods

Introduces new String methods such as startsWith, endsWith, and includes for easier string manipulation.

'hello'.startsWith('hell')

Number properties

Provides new static properties and methods on the Number object like isNaN and isFinite for improved number checking.

Number.isNaN(NaN)

Promises

Implements Promises for asynchronous programming, following the ES6 specification.

new Promise(function(resolve, reject) { resolve(42); })

Maps and Sets

Adds support for new collection types such as Map and Set, offering more options for data storage and manipulation.

new Map([[1, 'one'], [2, 'two']])

Other packages similar to es6-shim

core-js

A modular standard library for JavaScript, including polyfills for ECMAScript up to 2021. Offers broader coverage than es6-shim, including proposals not yet standardized.

babel-polyfill

Part of Babel, this package includes a custom regenerator runtime and core-js. It's more comprehensive but has been deprecated in favor of directly including core-js and the regenerator-runtime.

es5-shim

Focuses on ensuring ECMAScript 5 compatibility, providing shims for older JavaScript engines. It's more focused on ES5, making it complementary rather than directly comparable to es6-shim.

README

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.

Changelog

es6-shim 0.35.6 (14 October 2020)

• [Fix] Math.hypot: fix a bug in node v12.11 - v12.15
• [Fix] internal ToString operation should throw on Symbols
• [Robustness] call-bind Array.prototype.join
• [Refactor] String.raw: match latest spec
• [Refactor] address unused function args
• [readme] updating standalone shim links
• [Meta] Make LICENSE parseable by GitHub.
• [Dev Deps] update eslint, @ljharb/eslint-config, es5-shim, safe-publish-latest, evalmd
• [Tests] use shared travis-ci configs
• [Tests] only run prepublish when in publish
• [Tests] add aud in npm run posttest
• [Tests] add failing tests from https://github.com/tc39/test262/pull/2443
• [Tests] fix error message tests for Safari 12
• [Tests] fix sham function name tests