Utilizing a Function's Parameter Type in TypeScript: A Beginner's Guide

Currently, I am creating a custom method that wraps around Angular's HttpClient service. I want users to have the ability to pass in options, but I am struggling to find the proper way to reference that type in my method parameter definition.

For example:

  constructor(private http: HttpClient) {}

  my_request<T = any>(endpoint: string, payload: T, options: WhatTypeHere = {}) {
    return this.http.post<MyHttpResponse>(this.build_url(endpoint), this.build_payload(payload), {
      ...options,
      withCredentials: true,
    });
  }

In the options parameter of my_request(), I am unsure of what to specify in order to enforce it when using the method.

Here is the definition of HttpClient.post() for reference:

post(url: string, body: any | null, options?: {
        headers?: HttpHeaders | {
            [header: string]: string | string[];
        };
        observe?: 'body';
        params?: HttpParams | {
            [param: string]: string | string[];
        };
        reportProgress?: boolean;
        responseType?: 'json';
        withCredentials?: boolean;
    }): Observable<Object>;

I know that I could simply copy and paste the entire definition into my method, but I am looking for a more concise way to reference it, especially if definitions change in the future.

One approach I attempted was something like

options: HttpClient['post']['options']
, but it did not work as expected.

Is there an alternative method I can use to reference that type?

Answer №1

If you are working with at least typescript 3.3 (current version on the playground), it is now possible to extract the type of the 3rd parameter of the post function. Below is a simplified example to demonstrate this:

interface A {
    eh: number
}
function post(url: string, body: any | null, options?: {a: A}): void {}

// Creating an alias for convenience
type PostOptions = Parameters<typeof post>[2];

let options: PostOptions;
options = {}; // This should result in an expected error: Property 'a' is missing in type '{}' but required in type '{ a: A; }'.
options = { a: {} }; // This should also result in an expected error: Property 'eh' is missing in type '{}' but required in type 'A'.
options = {a: {eh: 10}} // This is the correct way to assign the values

Playground link

To provide a bit more insight, Parameters<T> represents the type of parameters of the function T as a tuple. Using typeof post, we obtain the function type and then extract the 3rd parameter type, which is at index 2 in the tuple.

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