How to fix 'type is not assignable to any' when addressing a member

February 9, 2023

I have a code like this:

interface IFoo {
  bar: string;
  baz: number;
}

function f(foo: IFoo, name: 'bar' | 'baz', val: any) {
  foo[name] = val;   // <<< error: Type 'any' is not assignable to type 'never'.
}

If I change the type of "baz" to be also "string" then the error is gone:

interface IFoo {
  bar: string;
  baz: string;
}

function f(foo: IFoo, name: 'bar' | 'baz', val: any) {
  foo[name] = val;   // fine
}

Why is this happening, and would it be possible to fix this?
I’m looking for a solution that is better than replacing name: 'bar' | 'baz' with name: string.

>Solution :

You must make sure, that the val has the correct type that corresponds to the provided name.

Typescript Playground Example

interface IFoo {
  bar: string;
  baz: number;
}

function f<K extends keyof IFoo>(foo: IFoo, name: K, val: IFoo[K]) {
  foo[name] = val;
}

const foo: IFoo = {
    bar: '',
    baz: 0
}

f(foo, 'bar', 'abc')
f(foo, 'baz', 1)
console.log(foo);