Improve Test Coverage

[zcaceres]

• Decide on a good, standard framework for testing in Typescript (vitest, jest, use bun and use bun:test?)
• Convert as many functions as possible to pure, deterministic functions
• Generate unit tests for these functions that cover reasonable edge cases

Markdownify could be refactored into a set of pure functions (no more class) and those functions could be good candidates for unit tests.

What makes a good candidate for a unit test?

• We don't want to test anything that is primarily logic in another library
• We don't want to test anything that includes "integration" logic such as HTTP calls, database writes, file system I/O etc. In these cases we want to extract out the elements that are "pure" (i.e. that do not make HTTP calls, db writes, file system I/O) and test those specifically.

Let's imagine we have a class like the following:

BEFORE:

import fetchMyApi from "myApi";

class TestClass {
   myMethod(param: string) {
       const response = await fetchMyApi(param);
       return response.json()
  } 
}

AFTER:

// EXPLICIT DEPENDENCY INJECTION FOR EVERYTHING THAT THE FUNCTION NEEDS
export async function myMethod(param: string, fetcher: FetcherFunction<MyResponse>) {
       const response = await fetcher(param);
       return response.json()
}

async function myMockFetch() {
   return {
     myMockPayloadProperty: 'hello world'
   }
}

test(myMethod('hello', myMockFetch), 'my expected output');

When we decide to test the function, we can use a mock to simulate the fetch rather than rely on the external dependency that's implicit.