Typescript mod functions don't allow type changes

[spiffytech]

Given this code:

shades.mod('prop')((prop: number) => prop.toFixed())

I get a type error saying Type 'string' is not assignable to type 'number', meaning the typings don't allow me to change the type of the property.

[jamesmcnamara]

You are completely correct.

The first versions of the typings allowed for type changes, but in practice this actually led to lots of bugs slipping through the cracks. When I removed this feature, it caught a ton of errors, but there were also basically no cases where I WAS transforming the type.

However, in the very few cases where I do some sort of type conversion, I have just preferred to use intersection types:

interface Obj {
  prop: string | number;
}

declare const obj: Obj;

shades.mod('prop')(prop => prop.toFixed())(obj)

If you really need to refine the type to a before and after type, you can use a type guard:

interface Obj {
  prop: string | number;
}

interface CleanedObj {
  prop: string;
}

function isCleanedObj(obj: Obj): obj is CleanedObj {
  return typeof obj.prop === 'string';
}

TL;DR: You're 100% right. This is a self-imposed limitation to try and catch more bugs.