What are the possible usage scenarios for the new JavaScript "Symbol" datatype?
javascript, symbols, types
Solution
After reading the documentation and playing a little bit with this `Symbol` type in chrome, it appears that a `Symbol` is a way to define a name -- not a value --, and the fact that properties defined using symbols are not visible using `for..in`, `Object.getOwnPropertyNames()` or `JSON.stringify()` makes symbols useful for metadata properties:
// define metadata symbols
var Metadata = {
Date: Symbol('Message date')
};
var email = function(recipient, message) {
this.Recipient = recipient;
this.Message = message;
this[Metadata.Date] = new Date();
};
var email1 = new email('@Me', 'test');
JSON.stringify(email1);
// {
// Recipient: '@Me',
// Message: 'test'
// }
// Date is still accessible using
email1[Metadata.Date];
// Thu Nov 27 2014 16:50:00 GMT+0000
// Debugging in Console:
// {
// Recipient: '@Me',
// Message: 'test'
// Symbol(Message date): Thu Nov 27 2014 16:50:00 GMT+0000
// }
Symbols can be made global using the `Symbol.for` function, so metadata names can be created once and used across all project files.
Accessing the value using a symbol requires having a reference to the symbol when created. Each call to `Symbol()` creates a new one even if the same description is used:
var a = Symbol('a');
var b = Symbol('a');
a != b
// and
a != Symbol('a')
but, creating a symbol using `Symbol.for`, it will be registered in a global registry and the description becomes a key, meanong only one symbol with the same key will exist in the global registry:
var a = Symbol.for('a');
var b = Symbol.for('a');
a == b
// and
a == Symbol.for('a')
Problem
I just stumbled upon the documentation for the new (proposed for ES6, but already implemented in Firefox, Chrome & Opera) datatype in JavaScript, `Symbol`: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol http://people.mozilla.org/~jorendorff/es6-draft.html#sec-symbol-objects I'm reading about it, but I just can't think of a possible usage scenario. The documentation says: A symbol is a unique and immutable data type and may be used as an identifier for object properties. OK, fine, let's say I do as the documentation says: ``` obj[Symbol("a")] = "a"; ``` but, since Symbol('a') always returns unique value (object) and: Symbols are not visible in for...in iterations. how do I retrieve my property from `obj`? ``` var obj = { normalProperty: 'just a string' }; obj[Symbol('a')] = 'a'; document.getElementById('retrieve').addEventListener('click', function() { document.write('Stringified object:' + JSON.stringify(obj) + '<br/><br/>'); document.write('Trying to get Symbol-property value, aaaand...: <br/>'); document.write(obj[Symbol('a')]); // undefined }, false); ``` ``` <button id="retrieve">Retrieve obj's property</button> ``` Edit Of course, you can retrieve it like this: ``` var x = Symbol('a'); obj[x] = 'a'; obj[x]; // "a" ``` but what's the purpose of doing it this way? Thanks in advance :)