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    Es6 Shim

    ECMAScript 6 (Harmony) compatibility shims for legacy JavaScript engines
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    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()
      • get()
      • getOwnPropertyDescriptor()
      • getPrototypeOf()
      • has()
      • isExtensible()
      • ownKeys()
      • 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.


    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');
    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,, NaN));
    assert.equal(true, -0 === 0);
    assert.equal(false,, 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.
    assert.equal(4, map.size);
    // Useful for storing unique items.
    var set = new Set([0, 1]);
    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));
    assert.equal(false, set.has(5));
    // Promises, see
    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


    • 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('') === will be true, which it would not by default in a fresh compliant realm.


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