fix(fetch): improve Headers and Request type-compatibility

[kettanaito]

• Reopened #1947 because GitHub UI is fantastic.

[kettanaito]

As mentioned in the previous review, a type test has to be added here. I will add it this/next week due to the work load.

[KhafraDev]

type tests are failing (npm run test:typescript)

   test/types/fetch.test-d.ts:141:0
  ✖  141:0  Parameter type string is declared too wide for argument type ReferrerPolicy.  

[MikeZeDev]

Suddenly after an update in my package i got the problem with the referrer property missing and i just found this library and this PR is related.

I manually edited fetch.d.ts to test this PR. I can get rid of the test error you mentioned if i revert

readonly referrerPolicy: ReferrerPolicy

to

readonly referrerPolicy: string

• Would that be acceptable ? I fear this is not strict enough or just a bad solution
• Should i make another PR because the person who submitted it seems to be busy?

[kettanaito]

@MikeZeDev, not sure if string isn't too broad for the referrer policy. It is annotated as RefererPolicy type in lib.dom, I think it'd be best if Undici introduced compatibility with that as well. Value-wise, you cannot set unknown values anyway. Can you tell me more about the use case that resulted in the error for you?

[kettanaito]

@KhafraDev, I've fixed the test by adjusting the expected type. It's no longer a wider string but a narrower ReferrerPolicy, which is compatible with how lib.dom annotates the referrerPolicy request property.

[kettanaito]

Type tests are passing now.

> [email protected] test:typescript
> tsd && tsc --skipLibCheck test/imports/undici-import.ts

@KhafraDev, may I please get the workflow approved on this pull request to see if there are any other pending things to resolve? Thanks.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.17%. Comparing base (e009b0c) to head (af9433a). Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1964   +/-   ##
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  Coverage   94.17%   94.17%           
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  Files          90       90           
  Lines       24434    24434           
=======================================
  Hits        23011    23011           
  Misses       1423     1423           

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[kettanaito]

@mcollina, note that this change can be technically breaking if consumers relied on referrerPolicy being a plain string. A narrower type shipped by this change will error on the type level in some cases.

My personal take on situations like this is if you rely on a bug, you should expect breaking changes. I'd release this as a patch fix since the type annotations were previously incorrect.