CODEWITHSUNDEEP
typescriptundefinedpartial
Sebastian Barth
Sebastian Barth

Reputation: 4551

Type similar to Record<A, B> but saying that not for every A there is a B value (not even an undefined)

Record is defined as follows:

/**
 * Construct a type with a set of properties K of type T
 */
type Record<K extends keyof any, T> = {
    [P in K]: T;
};

It is defined such that for a Record<A, B> for every A there is a B.

But how do I define that there might not be a B for every A?

If I use Partial<Record<A, B>> we would say for every A there is either B or undefined. But I don't want to except undefined as value:

type A = 'A1' | 'A2' | 'A3';


// Complains about "Type '{ A1: string; }' is missing the following properties from type 'B': A2, A3(2739)"
// But I want to be allowed to ommit any value of A
type B = Record<A, string>;
const fooB: B = {
    A1: 'value for A1'
}

// Allows to omit properties but also allows undefined as value, which I don't want.
type C = Partial<Record<A, string>>;
const fooC: C = {
    A1: 'value for A1',
    A2: undefined // Should not be allowed!
}

( Playground )

To disallow undefined makes a difference, for example when using Object.keys():

Object.keys({A2: undefined})
['A2']

It is still expected and fine that {}.A2 === undefined.

--exactOptionalPropertyTypes also can't be used as it would affect the whole codebase (If I am not mistaken also types from external dependencies used).

Upvotes: 0

Views: 92

Answers (1)

Nick Felker
Nick Felker

Reputation: 11978

It sounds like you want a type where every A is explicitly mapped to some output, but the output can be either B or undefined.

So you would want to use:

Record<A, B|undefined>

Upvotes: -1

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