Type symbol in Typescript

Alternative to string keys

symbol does not come up often in practice, symbol values are used as an alternative to string keys in objects and maps, in places where we want to be extra sure that people are using the right well-known key and didn’t accidentally set the key.

each Symbol is unique

We call Symbol(name) to create a new symbol with the given name. The way Symbol(name) works in Javascript is by creating a new symbol with the given name, that symbol is unique, and will not be equal(when compared with == or ===) to any other symbol(even if create a second symbol with the same name).

Think of Symbol(name) always creates a Symbol and append it to a list, and returns back the index of the Symbol.

let a1 = Symbol('a'); // symbol
let a2 = Symbol('a'); // symbol
console.log('a1 == a2', a1 == a2) // false
console.log('a1 === a2', a1 === a2) // false

think of unique symbols like other literal types such as 1, true

let a = Symbol('a'); // symbol
const b = Symbol('b'); // unique symbol
const c: unique symbol = Symbol('c'); // unique symbol
let d: unique symbol = Symbol('d');  // Error TS1332: A variable whose type is a `unique symbol` type must be 'const'.

1. When assignment a Symbol to a let or var variable, Typescript will infer its type as symbol, just like number(a)
2. When assignment a Symbol to a const variable, Typescript will infer its type as unique symbol(It will show up as symbol, not unique symbol), just like a number literal type 42(b)
3. We can explicitly annotate a const variable’s type as unique symbol, but not a let/var(c, d)