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Over the past few years of using Typescript and slowly getting comfortable writing complex custom types, I have amassed a collection of small helper types from around the internet. Here are a few of them that I end up adding to almost all my major projects and how I use them:
\nSometimes you need to get the resolved value of a Promise<T> wrapped type, mostly when using along with the inbuilt ReturnType type:
type Unpromise<T> = T extends Promise<infer U> ? U : T;\n\ntype a = Promise<number>;\ntype x = Unpromise<a>; // number\nUse with ReturnType to make ReturnTypeResolved:
type ReturnTypeResolved<T extends (...args: any) => any> = Unpromise<\n ReturnType<T>\n>;\n\nfunction test(): Promise<{ weird: \"value\" }> {\n return Promise.resolve({ weird: \"value\" });\n}\n\ntype WeirdType = ReturnTypeResolved<typeof test>;\n
I know we have the inbuilt Record but this is such a common and frequent use case that I like having this around:
type PlainObject<T = any> = Record<string, T>;\n\ntype NumMap = PlainObject<number>;\nconst a: NumMap = {\n key1: 1,\n key2: 2,\n keyx: \"x\", // ERROR: expected number\n};\nYou can also create a reverse of this, ValueOf<T>:
type ValueOf<T> = T[keyof T];\n\ntype x = ValueOf<NumMap>; // number\n
Partial but for nested objects, most useful when declaring defaults or merging some overrides for a config type:
type RecursivePartial<T> = {\n [P in keyof T]?: T[P] extends (infer U)[]\n ? RecursivePartial<U>[]\n : T[P] extends object\n ? RecursivePartial<T[P]>\n : T[P];\n};\n\nconst config = {\n a: {\n x: {\n p: 1,\n q: 2,\n },\n y: \"nested\",\n z: \"value\",\n },\n b: \"root\",\n c: \"value\",\n};\n\nconst configOverride: RecursivePartial<typeof config> = {\n a: {\n x: {\n p: \"wrong\", // should be number\n },\n },\n};\n
Go over the inbuilt utility types in the Typescript Docs. And checkout this great library, utility-types on Github for other common types people tend to use.
\nLinks to the various places I originally found these from:
\nUnpromise: Sourced from this StackOverflow Answer: https://stackoverflow.com/a/53196435/9485498RecursivePartial: Sourced from this StackOverflow Answer: https://stackoverflow.com/a/51365037/9485498