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ECMAScript 6 (Harmony) compatibility shims for legacy JavaScript engines
JavaScript HTML

ES6 Shim

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

Build Status dependency status dev dependency status

browser support

Sauce Test Status

HTML version of the final ECMAScript 6 spec


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.

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

Math functions’ accuracy is 1e-11.

  • Reflect

    • apply()
    • construct()
    • defineProperty()
    • deleteProperty()
    • enumerate()
    • get()
    • getOwnPropertyDescriptor()
    • getPrototypeOf()
    • has()
    • isExtensible()
    • ownKeys()
    • preventExtensions()
    • set()
    • setPrototypeOf()
  • String.prototype Annex B HTML methods 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:

    • anchor()
    • big()
    • blink()
    • bold()
    • fixed()
    • fontcolor()
    • fontsize()
    • italics()
    • link()
    • small()
    • strike()
    • sub()
    • sup()


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:


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 }


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 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

var assert = require('assert');

'abc'.startsWith('a'); // true
'abc'.endsWith('a'); // false
'john alice'.includes('john'); // true
'123'.repeat(2);    // '123123', NaN); // Fixes ===. 0 isnt -0, NaN is NaN
Object.assign({a: 1}, {b: 2}); // {a: 1, b: 2}

Number.isNaN('123'); // false. Global isNaN('123') will give true.
Number.isFinite('asd'); // false. Global isFinite() will give true.
// Tests if value is a number, finite,
// >= -9007199254740992 && <= 9007199254740992 and floor(value) === value
Number.isInteger(2.4); // false.

Math.sign(400); // 1, 0 or -1 depending on sign. In this case 1.

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

// 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(map.get(['meh']) === undefined); // undefined because you need to use exactly the same object.
assert(map.size === 4);

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

// Promises, see
Promise.resolve(5).then(function (value) {
  if (value) throw new Error("whoops!");
  // do some stuff
  return anotherPromise();
}).catch(function (e) {
  // any errors thrown asynchronously end up here


  • 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.


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