Symbols

A symbol is a primitive data type that represents a globally-unique value. You can create them with the Symbol function, which produces a value of type symbol.

Symbols are immutable and unique.

Just like strings, symbols can be used as keys for object properties.

Declaring unique symbols

A symbol will be treated as unique literal with the type annotation unique symbol. A value of type unique symbol can only be produced by calling Symbol() or Symbol.for(), or from explicit type annotations.

The unique symbol type is only allowed on const declarations and readonly static properties. To reference a specific unique symbol, use the typeof operator.

Keep in mind that each reference to a unique symbol implies a completely unique identity that is tied to a given declaration. Because each unique symbol has a completely separate identity, no two unique symbol types are assignable or comparable to each other.

Known symbols

There are several Symbol objects that are shared across the entire JavaScript execution environment.

• Symbol.toPrimitive
• Symbol.iterator