Symbol Type

Symbols

In JavaScript there is two primitive types that can be used as object property key

1. String
2. Symbol type

Don't confuse the symbol type in Ruby with JavaScript they are different albeit similar in that they are used to create unique identifier.

A symbol can be created using Symbol()

let id = Symbol();

You can give a symbol a description as it's parameter, mostly useful for debugging purposes.

// id is a symbol with description "this is id"
let id = Symbol("this is id");

Symbols are guaranteed to be unique, even if they have the same description, they are considered to be different values.

let id1 = Symbol("id");
let id2 = Symbol("id");

id1 == id2 // False

Property of symbols

• Symbols don't support implicit conversion to a String.

Hidden properties

Symbols allows you to create hidden properties on an object that no other part of code can accidentally access or overwrite

let user = {
	name: "John"
};

let id = Symbol("id");

user[id] = 1;

console.log(user[id]);

Here we are creating a hidden property using the id symbol that we have created. We can access it using the same symbol as the key.

However, if we tried to access it using String for example user["id"] we would get undefined.

Using symbol is to avoid conflict between say a third party code that wants to inject some of their own property into the object. If they are just using String key then it will likely overwrite some of the existing property for the object. However, if they use Symbols then there will be no conflicts between the identifiers.

Symbols... more

• You can use symbols in object literal, just use the square bracket when you are using symbols as key, like how you would use an variable as key
• Symbols are skipped in for ... in loops

Global symbols

If you want different named symbols to be referring to the same entity you can do that using global symbol registry. Create symbol in it and access them later, repeated access by the same name will return the same symbol.

The function Symbol.for(key) will look in the global registry for a key named key and return the Symbol that the key is mapped to. If it doesn't exist it will create one. Subsequent access to the same key will guaranteed to return the same Symbol.

let id = Symbol.for("id"); // read from global registry, create if it doesn't exist

let idAgain = Symbol.for("id"); // read it again in another part of code

id == idAgain // true

This symbol is the same symbol in Ruby.

Symbol.keyFor

If you want to find the key for a particular symbol using the Symbol object, you can do that with this function.

let globalSym1 = Symbol.for("name");
let globalSym2 = Symbol.for("name");

Symbol.keyFor(globalSym1); // "name"
Symbol.keyFor(globalSym2); // "name"