Cannot add string | number to string | number types in typescript

Question

So, I have type alias T which represents string | number union type

if I try to add two variables with this type, I'll get variable with type T. But if I take these two variables as parameters of a function it will be an error:

type T = string | number;

function add(a: T, b: T): T{
  return a + b; 
  // Error: Operator '+' cannot be applied to types 'T' and 'T'.
}

let a: T = 1;
let b: T = 'foo';

const c = a + b; // But this is ok

Why I can't do that?

Exact duplicate of typescript + operator usage on union types (which doesn't have an answer unfortunately). See also Operator '+' cannot be applied to types 'T' and 'T'. in TypeScript (where the titular T refers to a generic, not an alias, but the answer handles the same case) — Bergi
– Bergi, Commented May 29, 2021 at 15:58

Akhil · Accepted Answer · 2021-05-29 16:45:41Z

0

The error case here is number + number | string

You can verify that as the below function is not valid.

function addOne(x: number | string) {
  return 1 + x;
}

From the typescript github page

If T extends string | number then T can be string | number which means one value of type T can be string a different value of type T can be a number.

It really sounds like you should use overloads here rather than a type parameter.

edited May 29, 2021 at 16:45

answered May 29, 2021 at 16:18

Akhil

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1 Comment

abracatabra Over a year ago

But by that definition if both are numbers -- it's ok, if one number and one string -- it's ok. And when you trying to add number and number | string, only these two cases can happen. Seems like typescript not smart enough to figure that out

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