Symbol—A New Primitive Type

JavaScript primitive types

JavaScript previously had five primitive types:

1. number
2. string
3. Boolean
4. null
5. undefined

Now, it has one more type. SymbolSymbol is a new primitive type in JavaScript intended for limited specialized use.

The motivation to bring Symbol in ES6

Symbols can be used for three distinct purposes:

• Hidden properties: To define properties for objects in such a way they don’t appear during normal iteration. These properties are not private; they’re just not easily discovered like other properties.

• Global registry: To easily define a global registry or dictionary of objects.

• Special well-known Symbols: To define some special well-known methods in objects; this feature, which fills the void of interfaces, is arguably one of the most important purposes of Symbol.

Interfaces in languages like Java and C# are useful for design by contract and serve as a specification or a listing of abstract functions. When a function expects an interface, it is these languages guarantee that the object passed will conform to the interface’s specifications. However, there’s no such capability in JavaScript. You’ll see how Symbol helps fill the gap.

Let’s explore each of the benefits one by one with examples.