mapped-types

问题 I want to map an object type to a subtype that includes only keys whose values are of a specific type. For example, something like ExtractNumeric<T> , where ExtractNumeric<{ str: string, num: number }> should be equivalent to the type: { num: number } I've tried this, but it does not work: type ExtractNumeric<T> = { [k in keyof T]: T[k] extends number ? T[k] : never } This snippet throws a type error: let obj: ExtractNumeric<{ str: string, num: number }> = { num: 1 } Because although the str

问题 I want to map an object type to a subtype that includes only keys whose values are of a specific type. For example, something like ExtractNumeric<T> , where ExtractNumeric<{ str: string, num: number }> should be equivalent to the type: { num: number } I've tried this, but it does not work: type ExtractNumeric<T> = { [k in keyof T]: T[k] extends number ? T[k] : never } This snippet throws a type error: let obj: ExtractNumeric<{ str: string, num: number }> = { num: 1 } Because although the str

问题 I want to map an object type to a subtype that includes only keys whose values are of a specific type. For example, something like ExtractNumeric<T> , where ExtractNumeric<{ str: string, num: number }> should be equivalent to the type: { num: number } I've tried this, but it does not work: type ExtractNumeric<T> = { [k in keyof T]: T[k] extends number ? T[k] : never } This snippet throws a type error: let obj: ExtractNumeric<{ str: string, num: number }> = { num: 1 } Because although the str

问题 I use mongodb with @types/mongodb. This gives me a nice FilterQuery interface for my mogodb queries for a collection of shaped documents. In my domain object class I have some extra logic like converting dates to moment objects or floats to BigNumber objects. For my queries I need to convert these back, so for example a Moment object needs to be converted to a date object and so on. To avoid duplication and maintaining a separate interface (just for the queries) I thought of using mapped

问题 I'm trying to define helper types for determining the type of nested object values, whilst also considering any optional parent keys, e.g. in structures like these (or deeper): type Foo = { a: { b?: number; } }; type Foo2 = { a?: { b: number } }; For my purposes, the type of b in both Foo and Foo2 should be inferred as number | undefined . In Foo2 the b is not optional itself, but because a is, for my lookup purposes b must now be optional too... so much for context. Using these helper types

问题 Similar to this question I'm trying to create an object of a mapped type. However, I keep getting type errors in the implementation. Here's a toy example: type SingleArgFunction<A> = (x: A) => A; type ArrayedReturnFunction<A> = (x: A) => A[]; // create an arrayed function from a single arg function type MakeArrayed<F> = F extends SingleArgFunction<infer A> ? ArrayedReturnFunction<A> : never; // mapped type type MakeArrayedAll<FS> = { [K in keyof FS]: MakeArrayed<FS[K]> } // example usage type

问题 When optional and required property are merged via intersection, required wins type A = { who: string } type B = { who?: string } // $ExpectType {who:string} type R = A & B This may lead to runtime errors, when for instance, dealing with default params pattern within a function type Params = { who: string greeting: string } const defaults: Params = { greeting: 'Hello', who: 'Johny 5', } function greeting(params: Partial<Params>){ // $ExpectType Params const merged = {...defaults, ...params}

问题 When optional and required property are merged via intersection, required wins type A = { who: string } type B = { who?: string } // $ExpectType {who:string} type R = A & B This may lead to runtime errors, when for instance, dealing with default params pattern within a function type Params = { who: string greeting: string } const defaults: Params = { greeting: 'Hello', who: 'Johny 5', } function greeting(params: Partial<Params>){ // $ExpectType Params const merged = {...defaults, ...params}

问题 When optional and required property are merged via intersection, required wins type A = { who: string } type B = { who?: string } // $ExpectType {who:string} type R = A & B This may lead to runtime errors, when for instance, dealing with default params pattern within a function type Params = { who: string greeting: string } const defaults: Params = { greeting: 'Hello', who: 'Johny 5', } function greeting(params: Partial<Params>){ // $ExpectType Params const merged = {...defaults, ...params}

问题 When optional and required property are merged via intersection, required wins type A = { who: string } type B = { who?: string } // $ExpectType {who:string} type R = A & B This may lead to runtime errors, when for instance, dealing with default params pattern within a function type Params = { who: string greeting: string } const defaults: Params = { greeting: 'Hello', who: 'Johny 5', } function greeting(params: Partial<Params>){ // $ExpectType Params const merged = {...defaults, ...params}