Argument of type 'T' is not assignable to parameter of type 'string'

Question

I'm refactoring a JS code into TS (actually jsx/tsx) and I'm facing a problem with functions.

To simplify the issue... let's suppose I get this function "checkIt", which can receive a string or number as parameter and returns a boolean.

This is the JS function:

const FIRST_ROW = 'Initial';
const SECOND_ROW = new Set([1, 2, 3, 4]);

const checkIt = id => {
if (FIRST_ROW.includes(id) || SECOND_ROW.includes(id)) {
    return true;
  }
  return false;
}

Here is my trial... (as I'm learning TS, I like to declare inferred variables to be consistent)

const FIRST_ROW: string = 'Initial';
const SECOND_ROW: Set<number> = new Set([1, 2, 3, 4]);

const checkIt = <T,>(id: T): boolean => {
  if (FIRST_ROW.includes(id) || SECOND_ROW.includes(id)) {
    return true;
  }
  return false;
}

This gets me an error on my my "id" references inside the function: Argument of type 'T' is not assignable to parameter of type 'string' (or 'number'). I've tried using function expression, the Union string | number (as explained here), different forms of writing Generics, and Overloads (like here and I just can't make it work.

Any help here?? Thank you!

Poul Kruijt · Accepted Answer · 2021-04-23T18:40:00.400

4

How about using type casting? Just to hint the compiler:

const FIRST_ROW: string = 'Initial';
const SECOND_ROW: Set<number> = new Set([1, 2, 3, 4]);

const checkIt = (id: string | number): boolean => {
  if (FIRST_ROW.includes(id as string) || SECOND_ROW.has(id as number)) {
    return true;
  }
  return false;
}

playground

Also, Set does not have an include method.

To make it completely appropriate you first check if it's really a string. Because you use the union string|number the compiler knows that if typeof id === 'string' returns false, then id has to be a number, so casting is not necessary:

const FIRST_ROW: string = 'Initial';
const SECOND_ROW: Set<number> = new Set([1, 2, 3, 4]);

const checkIt = (id: string | number): boolean =>
  typeof id === 'string' ? FIRST_ROW.includes(id) : SECOND_ROW.has(id)
;

playground

edited Apr 23 '21 at 18:40

answered Apr 23 '21 at 18:25

Poul Kruijt

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Great! Thanks. About the *Set.includes* it was an error while writing it down here. – Анна Apr 23 '21 at 19:26
One question though @Poul ... Is it normal in TS to use plenties of *typeof* to constrain types? I've seen on the docs the use of it, but it didn´t sound natural to me, as it looked like I was overflowing my code with more text and logic that could be avoided with another way more simple and elegant... – Анна Apr 23 '21 at 19:30
1

@Анна if you have a union parameter, then you should definitely check before doing :) – Poul Kruijt Apr 23 '21 at 20:00

Argument of type 'T' is not assignable to parameter of type 'string'

1 Answers1