fix(http): negotiation returns the specific type of supported outcomes (when provided)

[Touffy]

This very simple PR adds a type parameter to all three negotiation functions (accepts, acceptsEncoding, acceptsLanguage) when they are called with more than one argument, so that the returned string will have the same specific type as the provided values.

This typing better reflects the actual behaviour (providing those arguments ensures that the returned value will be one of them, if any was acceptable) and improves type checking down the line when the content-types, encodings, or language code are used as keys (which is quite often, in my experience).

For example:

import { accepts } from "jsr:@std/[email protected]/negotiation"

const serializers = {
  "text/csv"() { return new TransformStream(/*...*/) },
  "application/x-ndjson"() { return new TransformStream(/*...*/) },
} satisfies Record<string, () => TransformStream<MyData, Uint8Array>>

type SupportedContentType = keyof typeof serializers

/* handling some request... */
Deno.serve((req) => {
  // currently, this is a string | undefined, but should be a SupportedContentType | undefined
  const acceptable = accepts(req, ...Object.keys(serializers))
  if (!acceptable) return new Response("Not Acceptable", {status: 406})

  const myData: ReadableStream<MyData> = getMyData()
  // because acceptable is a generic string, TypeScript doesn't trust it to access one of the serializers
  return new Response(myData.pipeThrough(serializers[acceptable]()))
})

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.11%. Comparing base (798fd7b) to head (3738b38).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6876   +/-   ##
=======================================
  Coverage   94.10%   94.11%           
=======================================
  Files         581      581           
  Lines       42699    42699           
  Branches     6790     6790           
=======================================
+ Hits        40182    40185    +3     
+ Misses       2466     2463    -3     
  Partials       51       51           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[kt3k]

Ok. Let's try this! LGTM