get-own-property-symbols

This is a widely compatible, mobile-friendly, and zero dependencies polyfill for Object.getOwnPropertySymbols.

var getOwnPropertySymbols = require('get-own-property-symbols');

var o = {};
var s = Symbol();

o[s] = 123;

Object.getOwnPropertyNames(o);  // []
getOwnPropertySymbols(o);       // [s]

// same as
Object.getOwnPropertySymbols(o);// [s]

This module brings in a global Symbol initializer too, together with Symbol.for and Symbol.keyFor methods.

var s = Symbol.for('me');
Symbol.for('me') === s; // true

Symbol.keyFor(s); // 'me'

Common symbols like iterator are also defined including the Array.prototype[Symbol.iterator] and the String.prototype[Symbol.iterator] method.

// this is the equivalent of a for/of in ES6
var iterator = [1,2,3][Symbol.iterator]();
var result;
while (!(result = iterator.next()).done) {
  console.log(result.value); // 1 then 2 and then 3
}


// this is the equivalent of a for/of in ES6
var iterator = '😺😲'[Symbol.iterator]();
var result;
while (!(result = iterator.next()).done) {
  console.log(result.value); // '😺' first and '😲' after
}

It is also possible to simply copy same iterator for any other iterable collection.

Caveats

There are few things developers need to know about Symbol partial polyfills. Here a quick summary.

null Objects

This polyfill will not work with null objects, and even if it's possible to make it work it's not worth the hassle.

var o = Object.create(null); // or {__proto__: null}
var s = Symbol();

o[s] = 123;

// not set as Symbol, just as generic key
Object.keys(o); // [s]

the typeof gotcha

It is not possible to overwrite native typeof operator and while it returns symbol with native support, since version 0.5.0 it returns object when polyfilled. This is not perfect, but at least it's simple to distinguish between Symbols and regular properties in list of mixed properties collections.

cross-realm incompatibility

Symbol.for and Symbol.keyFor can't be shimmed cross-realm. To be extra fair, Symbol should never be used cross-realm unless natively supported.

is Symbol native ?

Since it's not possible to overwrite typeof, a check against typeof key === "symbol" is all we need to understand if support is native or not. Please note that transpilers might wrap this check so we should be sure the test is done natively and not before transpiling.

the in operator

Since it's also not possible to overwrite in, please note that Symbol() in {} is always true since SYmbols need to be shimmed through the Object.prototype.

How to use

Either npm install get-own-property-symbols or include this file on your page.

More details

There are alternatives to this polyfill Symbol only and the main difference is that whit get-own-property-symbols you actually have Object.getOwnPropertySymbols functionality and Object.getOwnPropertyNames will never show Symbols too.

Also today core-js brings Symbols in, but as part of the entire core-js partial polyfill, and with same caveats described in here.

Accordingly, if you are looking for a backward compatible, stand-alone version, as ES6 compliant as possible partial polyfill, use this module, otherwise feel free to pick alternatives.

Please note this polyfill is also compatible with Object.assign.

Compatibility

Mobile

• Android 2.x and higher ( Android 2 requires code minification if Symbol.for is used, or Symbol['for'] instead )
• iOS5 and higher
• Windows Phone 7 (IE9 Mobile) and higher
• Blackberry OS7 and higher
• FirefoxOS 1.0 and higher
• Opera Mini and Opera Mobile
• Ubuntu Phone, Kindle Fire, and all others based on Webkit or Chrome
• yes, even Palm WebOS 2 works

Desktop

• IE9 and higher
• Chrome and Opera
• Firefox
• Safari

Server

• node js 0.6 and higher
• io.js has native support, so it works there too
• Duktape and Nashorn should be fine too (please let me know if not)