Using symbol as object-key type in TypeScript

Question

I'm trying to define an object with a symbol as key-type since MDN says:

A symbol value may be used as an identifier for object properties [...]

But using it as type for the key-property:

type obj = {
    [key: symbol | string]: string
}

results in the following error:

TS1023: An index signature parameter type must be either 'string' or 'number'.

even it can be used as index-type. I'm using the latest typescript version (v3.7.2), related questions I've found:

• Typescript: destructuring an object with symbols as keys (He's using an actual instance of a Symbol, I want the type symbol)
• TypeScript: An index signature parameter must be a 'string' or 'number' when trying to use string | number
• ES6: destructuring an object with symbols as keys (That can't be a solution - it seems kinda wrong to use an actual instance as type since every Symbol instance is unique...)

I've also took a look at the typescript symbol docs but they only show how it's used as value, not as type.

Example:

const obj = {} as {
    [key: number | symbol]: string // Won't work
};

const sym = Symbol('My symbol');
obj[sym] = 'Hi';

Issue on Microsoft/TypeScript

Open feature request

Answer 1

TypeScript 4.4 allows symbols in index signatures:

type SymbolIndex = {
    [key: symbol | string]: string // works
}

const sym = Symbol("descr");
const t1: SymbolIndex = {
    "foo": "bar",
    [Symbol.iterator]: "qux",
    sym: "sym"
};

// all result in string
t1.foo 
t1.sym 
t1[Symbol.iterator]
t1["oh"]

Playground

With older versions, SymbolIndex will trigger an error:

An index signature parameter type must be either 'string' or 'number'.(1023)

Alternative

If you just want an object type with symbols and no index signature, you can already do that today:

const sym = Symbol() // note const (no let) 
type O = {
    foo: string
    [Symbol.iterator]: string
    [sym]: number
}

let o: O = { [sym] : 3, [Symbol.iterator]: "bar", foo: "qux"}

let { [sym]: symVal } = o

Playground