Why does TypeScript complain when calculating numbers?

Question

Let's say I have a constant number defined, eg:

const a = 3;

Why does TypeScript complain about this?

const b: 10 = a * 3 + 1;

Type 'number' is not assignable to type '10'

In the generic case, I'd like b to have a defined set of allowed values, eg 10 | 16 | 19 | 25 10 | 16 | 19 | 25 10 | 16 | 19 | 25 , and I'd like TypeScript to allow:

const a = 3; // or 5, 6, 8

and error with all other a s.

Is this possible?

Answer 1

When you specify the following line

const b: 10 = a * 3 + 1;

It means the const b will be a type of 10, which is not.

For the following line

const a = 3; // or 5, 6, 8

You will need to set the type to be as -

const a: 3 | 5 | 6 | 8 = 3;

However, you can also pull it off to be a type for reusable