Generic function Type 'number' is not assignable to type 'U'

November 2, 2022

I am getting this error, Where as U is number and returning the number too.

Type 'number' is not assignable to type 'U'.

function foo<T extends string, U extends number>(a: T): U {
    return +a; // error in this line
}


foo<string, number>('12')

>Solution :

I believe it worth using overloading here:

type UnsafeNumber = number & { tag?: 'UnsafeNumber' }

type ParseInt<Str extends string> =
  Str extends `${infer Digit extends number}`
  ? Digit
  : UnsafeNumber


function foo<T extends string>(a: T): ParseInt<T>
function foo<T extends string>(a: T) {
  return +a;
}

type Result = ParseInt<'.2'>

const _ = foo('42') // 42
const __ = foo('4.2') // 4.2
const ___ = foo('s03') // UnsafeNumber

Playground

Since TS does not distinguish number and NaN I have provided UnsafeNumber which corresponds to NaN. However, it is still not safe and I would recommend you to use parseItn function