Except is not strict

[nurliman]

import { Except } from "type-fest";

type Foo = {
    a: number;
    b: string;
    c: boolean;
};

type FooWithoutA = Except<Foo, 'a'>;

const foo: Foo = {
    a: 1,
    b: 'b',
    c: true
}

const fooWithoutA: FooWithoutA = foo; // not error

playground link

Is this intended behavior? if yes, how can i restrict foo for being assign to fooWithoutA?
Thank you.

[orimiles5]

This is probably because TypeScript has a design decision that causes it to only check for excess properties when an object is defined using an object literal syntax.

It is possible to restrict foo for being assign to fooWithoutA with this solution:

import { Except } from "type-fest";

type StrictExcept<T, K extends keyof T> = Except<T, K> & Partial<Record<K, never>>;

type Foo = {
    a: number;
    b: string;
    c: boolean;
};

type FooWithoutA = StrictExcept<Foo, 'a'>;

const foo: Foo = {
    a: 1,
    b: 'b',
    c: true
}

const fooWithoutA: FooWithoutA = foo; // error

If relevant, I would be happy to submit a PR and fix the original Except.

[nurliman]

This is probably because TypeScript has a design decision that causes it to only check for excess properties when an object is defined using an object literal syntax.

It is possible to restrict foo for being assign to fooWithoutA with this solution:

import { Except } from "type-fest";

type StrictExcept<T, K extends keyof T> = Except<T, K> & Partial<Record<K, never>>;

type Foo = {
    a: number;
    b: string;
    c: boolean;
};

type FooWithoutA = StrictExcept<Foo, 'a'>;

const foo: Foo = {
    a: 1,
    b: 'b',
    c: true
}

const fooWithoutA: FooWithoutA = foo; // error

If relevant, I would be happy to submit a PR and fix the original Except.

Cool, your solution worked. Thank you!

[tommy-mitchell]

It is possible to restrict foo for being assign to fooWithoutA with this solution:

// ...

If relevant, I would be happy to submit a PR and fix the original Except.

@orimiles5 Maybe adding an ExceptOptions similar to Get would suffice?

type-fest/source/get.d.ts

Lines 5 to 14 in a065865

	type GetOptions = {
	/**
	Include `undefined` in the return type when accessing properties.
	Setting this to `false` is not recommended.
	@default true
	*/
	strict?: boolean;
	};

The default behavior would be the updated strict version, and users could opt in to the non-strict Except if they were relying on the old behavior.

[orimiles5]

It is possible to restrict foo for being assign to fooWithoutA with this solution:

// ...

If relevant, I would be happy to submit a PR and fix the original Except.

@orimiles5 Maybe adding an ExceptOptions similar to Get would suffice?

type-fest/source/get.d.ts

Lines 5 to 14 in a065865

	type GetOptions = {
	/**
	Include `undefined` in the return type when accessing properties.
	Setting this to `false` is not recommended.
	@default true
	*/
	strict?: boolean;
	};

The default behavior would be the updated strict version, and users could opt in to the non-strict Except if they were relying on the old behavior.

Great idea. I’ll work on it. Thank you!