Typescript: get keys from Map or object

Question

Let's say I want to write a function that returns the keys from either a Map or a plain object as a plain array, something like this:

function keys <K>(m: Map<K, any> | { [key: string]: any }) {
  if (m instanceof Map) return Array.from(m.keys())
  return Object.keys(m)
}

If I then try to retrieve the keys from a normal object, that works fine:

let a = ((g: string[]) => g)(keys({})) // this works

However, using a Map<number, number> gives me a type error:

let a = ((g: number[]) => g)(keys(new Map<number, number>()))
// Argument of type 'number[] | string[]' is not assignable to parameter of type 'number[]'

let a = ((g: number[]) => g)(keys<number>(new Map<number, number>()))
// Even this produces the same error

let a = ((g: number[]) => g)(keys(new Map<number, number>()) as number[])
// This works but the cast kind of defeats the purpose

What am I missing? How should I type this to make the TypeScript compiler understand what I mean?

Update:

Function signature overloading seems to be the way to go here, this version works fine:

function keys <K>(m: Map<K, any>): K[]
function keys (m: { [key: string]: any }): string[]
function keys (m: any): any {
  if (m instanceof Map) return Array.from(m.keys())
  return Object.keys(m)
}

Answer 1

Check typescript function signature overloading: https://www.typescriptlang.org/docs/handbook/functions.html#overloads

you can define a set of different signatures, so you can define a signature for the Map case, and one for the object case:

function keys <K>(m: Map<K, any>): K[]
function keys (m: { [key: string]: any }): string[]
function keys (m: any): any {
  if (m instanceof Map) return Array.from(m.keys())
  return Object.keys(m)
}

Answer 2

You can use plain functions with generics:

function keys<K extends PropertyKey>(m: Map<K, any> | { [P in K]: any }): K[] {
    if (m instanceof Map) return Array.from(m.keys())
    return Object.keys(m) as K[]
}

let a = (g => g)(keys({})) // never[]
let a0 = (g => g)(keys({foo: "bar"})) // "foo"[]
let a1 = (g => g)(keys(new Map<number, number>())) // number[]
let a2 = (g => g)(keys<number>(new Map<number, number>())) // number[]
let a3 = (g => g)(keys(new Map<1 | 2, number>())) // (1 | 2)[]

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