86 lines
2.0 KiB
Plaintext
86 lines
2.0 KiB
Plaintext
import type {KeysOfUnion} from './keys-of-union';
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/**
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Pick keys from a type, distributing the operation over a union.
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TypeScript's `Pick` doesn't distribute over unions, leading to the erasure of unique properties from union members when picking keys. This creates a type that only retains properties common to all union members, making it impossible to access member-specific properties after the Pick. Essentially, using `Pick` on a union type merges the types into a less specific one, hindering type narrowing and property access based on discriminants. This type solves that.
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Example:
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```
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type A = {
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discriminant: 'A';
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foo: {
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bar: string;
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};
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};
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type B = {
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discriminant: 'B';
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foo: {
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baz: string;
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};
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};
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type Union = A | B;
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type PickedUnion = Pick<Union, 'discriminant' | 'foo'>;
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//=> {discriminant: 'A' | 'B', foo: {bar: string} | {baz: string}}
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const pickedUnion: PickedUnion = createPickedUnion();
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if (pickedUnion.discriminant === 'A') {
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// We would like to narrow `pickedUnion`'s type
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// to `A` here, but we can't because `Pick`
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// doesn't distribute over unions.
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pickedUnion.foo.bar;
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//=> Error: Property 'bar' does not exist on type '{bar: string} | {baz: string}'.
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}
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```
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@example
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```
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type A = {
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discriminant: 'A';
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foo: {
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bar: string;
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};
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extraneous: boolean;
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};
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type B = {
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discriminant: 'B';
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foo: {
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baz: string;
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};
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extraneous: boolean;
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};
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// Notice that `foo.bar` exists in `A` but not in `B`.
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type Union = A | B;
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type PickedUnion = DistributedPick<Union, 'discriminant' | 'foo'>;
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const pickedUnion: PickedUnion = createPickedUnion();
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if (pickedUnion.discriminant === 'A') {
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pickedUnion.foo.bar;
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//=> OK
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pickedUnion.extraneous;
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//=> Error: Property `extraneous` does not exist on type `Pick<A, 'discriminant' | 'foo'>`.
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pickedUnion.foo.baz;
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//=> Error: `bar` is not a property of `{discriminant: 'A'; a: string}`.
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}
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```
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@category Object
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*/
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export type DistributedPick<ObjectType, KeyType extends KeysOfUnion<ObjectType>> =
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ObjectType extends unknown
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? Pick<ObjectType, Extract<KeyType, keyof ObjectType>>
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: never;
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