Typescript Advance Use Cases

Some use cases of how those type helps.

Type value filter

Assume we have a fixed object like this, and we want GET only records


type AppRouteMap = {
    "/users/dummyTest": "GET";
    "/users/checkAvaiableUser": "POST";
    "/files/uploadSignatureTest": "POST";
    "/files/uploadDocsTest": "POST";
    "/testing/dummyTest": "GET";
}
 
type OnlyGetRoutes = {
    [K in keyof AppRouteMap as AppRouteMap[K] extends "GET" ? K : never]: AppRouteMap[K]
};
// type OnlyGetRoutes = {
//     "/users/dummyTest": "GET";
//     "/testing/dummyTest": "GET";
// }
 
export type RoutersPathLessStrictOnlyGet = `${keyof OnlyGetRoutes}${string}`;
// type RoutersPathLessStrictOnlyGet = `/users/dummyTest${string}` | `/testing/dummyTest${string}`

This time we keep keys that start with “/files/”:


type FilesOnly = {
    [K in keyof AppRouteMap as K extends `/files/${string}` ? K : never]: AppRouteMap[K]
};
// {
//     "/files/uploadSignatureTest": "POST";
//     "/files/uploadDocsTest": "POST";
// }