mirror of
https://github.com/sindresorhus/type-fest.git
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73 lines
3.0 KiB
TypeScript
73 lines
3.0 KiB
TypeScript
import type {Except} from './except.d.ts';
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import type {Simplify} from './simplify.d.ts';
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/**
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Create a writable version of the given array type.
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*/
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type WritableArray<ArrayType extends readonly unknown[]> =
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ArrayType extends readonly [] ? []
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: ArrayType extends readonly [...infer U, infer V] ? [...U, V]
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: ArrayType extends readonly [infer U, ...infer V] ? [U, ...V]
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: ArrayType extends ReadonlyArray<infer U> ? U[]
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: ArrayType;
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/**
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Create a type that strips `readonly` from the given type. Inverse of `Readonly<T>`.
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The 2nd argument will be ignored if the input type is not an object.
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Note: This type can make readonly `Set` and `Map` writable. This behavior is different from `Readonly<T>` (as of TypeScript 5.2.2). See: https://github.com/microsoft/TypeScript/issues/29655
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This can be used to [store and mutate options within a class](https://github.com/sindresorhus/pageres/blob/4a5d05fca19a5fbd2f53842cbf3eb7b1b63bddd2/source/index.ts#L72), [edit `readonly` objects within tests](https://stackoverflow.com/questions/50703834), [construct a `readonly` object within a function](https://github.com/Microsoft/TypeScript/issues/24509), or to define a single model where the only thing that changes is whether or not some of the keys are writable.
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@example
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```
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import type {Writable} from 'type-fest';
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type Foo = {
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readonly a: number;
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readonly b: readonly string[]; // To show that only the mutability status of the properties, not their values, are affected.
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readonly c: boolean;
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};
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const writableFoo: Writable<Foo> = {a: 1, b: ['2'], c: true};
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writableFoo.a = 3;
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// @ts-expect-error
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writableFoo.b[0] = 'new value'; // Will still fail as the value of property "b" is still a readonly type.
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writableFoo.b = ['something']; // Will work as the "b" property itself is no longer readonly.
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type SomeWritable = Writable<Foo, 'b' | 'c'>;
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// type SomeWritable = {
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// readonly a: number;
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// b: readonly string[]; // It's now writable. The type of the property remains unaffected.
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// c: boolean; // It's now writable.
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// }
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// Also supports array
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const readonlyArray: readonly number[] = [1, 2, 3];
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// @ts-expect-error
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readonlyArray.push(4); // Will fail as the array itself is readonly.
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const writableArray: Writable<typeof readonlyArray> = readonlyArray as Writable<typeof readonlyArray>;
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writableArray.push(4); // Will work as the array itself is now writable.
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```
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@category Object
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*/
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export type Writable<BaseType, Keys extends keyof BaseType = keyof BaseType> =
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BaseType extends ReadonlyMap<infer KeyType, infer ValueType>
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? Map<KeyType, ValueType>
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: BaseType extends ReadonlySet<infer ItemType>
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? Set<ItemType>
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: BaseType extends readonly unknown[]
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// Handle array
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? WritableArray<BaseType>
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// Handle object
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: Simplify<
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// Pick just the keys that are not writable from the base type.
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Except<BaseType, Keys> &
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// Pick the keys that should be writable from the base type and make them writable by removing the `readonly` modifier from the key.
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{-readonly [KeyType in keyof Pick<BaseType, Keys>]: Pick<BaseType, Keys>[KeyType]}
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>;
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export {};
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