Symbols

Symbols

A symbol is a unique and immutable data type (added in ES6).

The Symbol object is an implicit object wrapper for the symbol primitive data type.

Symbol([description])

MDN // Symbol

A symbol is not an object, but a primitive type.

var sym1 = Symbol()
var sym2 = Symbol('foo')

console.log(sym1) // Symbol()
console.log(sym2) // Symbol(foo)
console.log(typeof sym1) // "symbol"

Symbol("foo") does not coerce the string "foo" into a symbol.

It creates a new symbol each time

Symbols are always unique.

var sym1 = Symbol('foo')
var sym2 = Symbol('foo')
var obj = { [sym1]: 1, [sym2]: 2 }

sym1 == sym2 // false
sym1 === sym2 // false
obj[sym1] // 1
obj[sym2] // 2

Symbol is a function.

typeof Symbol // "function"

As a function, Symbol is an object with properties.

Many of them are simply global constants.

Symbol = {
  /* ... */
  hasInstance: Symbol(Symbol.hasInstance)
  iterator: Symbol(Symbol.iterator)
  search: Symbol(Symbol.search)
  toPrimitive: Symbol(Symbol.toPrimitive)
  toStringTag: Symbol(Symbol.toStringTag)
}

...that we may talk about later on...

Symbol is not a constructor.

new Symbol('foo')
// Uncaught TypeError: Symbol is not a constructor