symbol

This component implements ECMAScript 6-like Symbols in ECMAScript 5. A Symbol can be used as a unique key for setting non-enumerable (i.e. "invisible") properties on objects. This is specially useful for implementing truly private stores in JS objects. This component can be used both on its own or as a Polyfill for ES6 Symbols.

var symbol = require('symbol');

var obj = { name: 'John Doe' };

(function() {
  var privateStore = symbol();

  obj[privateStore] = {
    secretAgent: true,
    realName: 'James Bond'
  }
})();

// this will not expose privateStore
for (var key in obj) {
  console.log(key);
}

Installation

Install with component(1):

$ component install component/symbol

API

symbol()

Creates a new symbol.

Symbol#dispose()

Disposes the symbol, releasing the global resources used by it.

This is only really needed when using a large number of short-lived, disposable Symbol objects. Please refer to the "Implementation Considerations" and "Memory Leak" sections below for more details.

Usage as a polyfill for native Symbols

if (typeof window.Symbol == 'undefined') {
  window.Symbol = require('symbol');
}

Implementation Considerations

The implementation is based on an example on the WebReflection blog, by Andrea Giammarchi:

http://webreflection.blogspot.com/2013/03/simulating-es6-symbols-in-es5.html

(The original example implementation is MIT licensed, and so is this component.)

The "trick" for getting a non-enumerable property when using a Symbol as a key — without having to manually resort to Object.defineProperty() every time — is defining for each symbol a property on the global Object prototype, that's responsible for calling Object.defineProperty() for us when set. This is not as bad as it sounds, as the property is both unique (using https://github.com/component/uid) and non-enumerable.

Memory Leak

Important: While collisions or changes in behavior will not be triggered by the presence of the global property for the reasons stated above, its value will not be automatically released, so this specific implementation will by default leak memory over time if used for a very large number of disposable Symbol objects.

If you need to use a large number of disposable Symbol objects, make sure to manually call the dispose() method on each of them before disposing them.

License

The MIT License (MIT)

Copyright (c) 2013 WebReflection / Andrea Giammarchi Copyright (c) 2014 Automattic, Inc.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.