Except is broken when used with generic property

[szmarczak]

class Example {
    foo: number;
    bar: string;

    constructor() {
        this.foo = 1;
        this.bar = '';
    }

    [key: string]: unknown;
}

type Except<ObjectType, KeysType extends keyof ObjectType> = Pick<ObjectType, Exclude<keyof ObjectType, KeysType>>;
const test: Except<Example, 'bar'> = new Example();

// `foo` is now `unknown`
console.log(test.foo + 5);

Playground link

[szmarczak]

I just noticed Omit is broken as well 🤦🏼

[sindresorhus]

This sounds like a TS bug/limitation. Maybe open an issue about Omit and see what they say?

[szmarczak]

microsoft/TypeScript#45177

[szmarczak]

type IfPrimitiveString<C, T, F> = string extends C ? T : F;
type EqualsTest<T> = <A>() => A extends T ? 1 : 0;
type Equals<A1, A2> = EqualsTest<A2> extends EqualsTest<A1> ? 1 : 0;

type Filter<K, I> = Equals<K, I> extends 1 ? never : K;

type OmitIndex<T, I extends string | number> = {
  [K in keyof T as Filter<K, I>]: T[K];
};

[szmarczak]

I just wanted to note that is Except breaks everything that uses it, including Merge.

[sindresorhus]

Can we make a better Except type here?

[szmarczak]

We can, but under a different name. Some people want Except<Example, string> to remove all string keys and some will want to remove only generic string keys.

[szmarczak]

I'll do a PR.

[sindresorhus]

What would you name such a type?

[szmarczak]

Maybe ExceptStrict?

[nghiepdev]

The problem reappeared in 2.5.2.

[sindresorhus]

I don't see how that's possible. Nothing related to Except was touched in 2.5.2.