I am facing a challenge while trying to determine the return type for the function post. The function's second parameter is an object that contains a transform property. If the transform parameter is provided, then the return type of post should be the same as the return type of transform. Otherwise, it should default to the first generic type provided for the post function. I hope this explanation is clear.
type DefaultHttpCallInit = {
throttle?: number;
baseUrl?: string;
url?: string;
onSend?(e: HttpCallInit): void | Promise<void>;
}
export type HttpCallInit = RequestInit & DefaultHttpCallInit
export type HttpCallInitOf<T> = RequestInit & DefaultHttpCallInit & {
transform?: <TOut>(v: T) => TOut
};
export type HttpCallerInstance = {
post<T, TInit extends HttpCallInitOf<T>>(data?: T, init?: TInit): Promise<TInit extends {transform(e: T): infer XOut} ? XOut : T>;
}
//hack reference
let r = {} as HttpCallerInstance;
interface Post {
id?: number;
title: string;
}
interface User {
id: number;
userName: string;
}
interface UserPost extends Post{
user: User
}
const user = {/* info props */} as User;
r.post({title: 'New post'} as Post, {
//First: infer
transform(post) {
return Object.assign(post, {user}) as UserPost
}
})
{
"compilerOptions": {
"strict": true,
"noImplicitAny": true,
"strictNullChecks": true,
"strictFunctionTypes": true,
"strictPropertyInitialization": true,
"strictBindCallApply": true,
"noImplicitThis": true,
"noImplicitReturns": true,
"alwaysStrict": true,
"esModuleInterop": true,
"declaration": true,
"experimentalDecorators": true,
"emitDecoratorMetadata": true,
"target": "Latest",
"module": "ESNext",
"moduleResolution": "node"
}
}
Try it out on Typescript Playground
Here is the error message I encountered:
https://i.sstatic.net/tZXEx.png