conditional-types

问题 I have a TypeScript code snippet in the playground. Please take a look there at TypeScript playground or here: enum MyTypes { FIRST = "FIRST", SECOND = "SECOND", THIRD = "THIRD" } type TFirst = { type: MyTypes.FIRST foo: string } type TSecond = { type: MyTypes.SECOND foo: string } type TThird = { type: MyTypes.THIRD bar: string } type TConditionalType<T> = T extends MyTypes.FIRST ? TFirst : T extends MyTypes.SECOND ? TSecond : T extends MyTypes.THIRD ? TThird : null const

问题 I have a TypeScript code snippet in the playground. Please take a look there at TypeScript playground or here: enum MyTypes { FIRST = "FIRST", SECOND = "SECOND", THIRD = "THIRD" } type TFirst = { type: MyTypes.FIRST foo: string } type TSecond = { type: MyTypes.SECOND foo: string } type TThird = { type: MyTypes.THIRD bar: string } type TConditionalType<T> = T extends MyTypes.FIRST ? TFirst : T extends MyTypes.SECOND ? TSecond : T extends MyTypes.THIRD ? TThird : null const

问题 I have a TypeScript code snippet in the playground. Please take a look there at TypeScript playground or here: enum MyTypes { FIRST = "FIRST", SECOND = "SECOND", THIRD = "THIRD" } type TFirst = { type: MyTypes.FIRST foo: string } type TSecond = { type: MyTypes.SECOND foo: string } type TThird = { type: MyTypes.THIRD bar: string } type TConditionalType<T> = T extends MyTypes.FIRST ? TFirst : T extends MyTypes.SECOND ? TSecond : T extends MyTypes.THIRD ? TThird : null const

问题 I have a TypeScript code snippet in the playground. Please take a look there at TypeScript playground or here: enum MyTypes { FIRST = "FIRST", SECOND = "SECOND", THIRD = "THIRD" } type TFirst = { type: MyTypes.FIRST foo: string } type TSecond = { type: MyTypes.SECOND foo: string } type TThird = { type: MyTypes.THIRD bar: string } type TConditionalType<T> = T extends MyTypes.FIRST ? TFirst : T extends MyTypes.SECOND ? TSecond : T extends MyTypes.THIRD ? TThird : null const

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 Assume I have some a-lot-of-union type: var MyComplexType = MyType1 | MyType2 | MyType3 | ... | MyTypeN where MyType{N} has this kind of signature: type MyType1 = { type: string, data: <different data for different types> } I know that I can use a kind of type guard function, e. g.: function isMyComplexTypeOfMyType1(item: MyComplexType): item is MyType1 { return item.type == "type of MyType1" } but in this case I should write a lot of this kind of functions. So, the question is: can I