feat(fetch): introduce HttpMethod type and update RequestInit method typing

[plvo]

This relates to...

This PR enhances the developer experience by adding proper TypeScript typing for HTTP methods and missing RequestInit properties to match the standard Fetch API specification. https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Methods

Added a new HttpMethod type to define valid HTTP methods and updated the method property in RequestInit to use this type. Updated tests to reflect these changes.

Rationale

Changes

Enhanced TypeScript Definitions

• Added HttpMethod type: Union of standard HTTP methods + extensible string type using (string & {}) pattern
• Updated RequestInit.method: Now typed as HttpMethod instead of generic string

Test Coverage

• Added comprehensive TypeScript tests for all new properties and types
• Enhanced method testing to verify HttpMethod typing works correctly
• Maintained backward compatibility - all existing code continues to work

Features

• HTTP Method Autocompletion: IntelliSense now suggests standard HTTP methods (GET, POST, PUT, DELETE, HEAD, OPTIONS, PATCH, TRACE, CONNECT)
• Extensible Method Typing: Custom HTTP methods still work via (string & {}) pattern
• Improved Developer Experience: Better type safety and IDE support

Bug Fixes

Breaking Changes and Deprecations

None - This is a purely additive change that maintains full backward compatibility. All existing code will continue to work without modification.

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

[tsctx]

Breaking Changes and Deprecations
None - This is a purely additive change that maintains full backward compatibility. All existing code will continue to work without modification.

Isn't this a breaking change? It won't be possible to merge of declarations of the following declare module 'undici' {interface RequestInit {method?: string;}} .

[plvo]

@tsctx You're right about the issue. I've found this is a fundamental TS limitation after testing:

• method?: string → Module augmentation works but no autocompletion
• method?: HttpMethod → Autocompletion works but breaks module augmentation

But it works like this

import "undici"
import type { HttpMethod } from "undici";

declare module "undici" {
  interface RequestInit {
    method?: HttpMethod;
  }
}

[Uzlopak]

Maybe do it like we do it in fastify?

https://github.com/fastify/fastify/blob/main/types/utils.d.ts

Also I realize now, you could force uppercase strings for HttpMethod.

https://www.typescriptlang.org/play/?#code/KYDwDg9gTgLgBDAnmYcASMZgLLBgCwgBM4BeOAcgHEBRAFQrgB9K0aBBAEUZYoAUA8gGUGzSnwCqo3pxoAZejR6UAwgIBy6miumUBfOgEkNQ5RToAldiqVj+7OirTKAFBLAooAYwCGAZ2AAHj8YKABLADsAcwA+OAAyOAtgL2giQIjgADdgKAAaOEycqBiASgBYACgq1IiQuAAjABsfAC50TBw8QiIAbQBdMjheigAhOQlRigKKfyIKfqA

[plvo]

@Uzlopak good idea to use Autocomplete, the code is cleaner that way :

export type HTTPMethods =
  | 'GET'  | 'HEAD'  | 'POST'  | 'PUT'  | 'DELETE'  | 'CONNECT'  | 'OPTIONS'  | 'TRACE'  | 'PATCH'

export type HTTPMethod = Autocomplete<HTTPMethods | Lowercase<HTTPMethods>>

export interface RequestInit {
  method?: HTTPMethod
  // ...
}

however, there's still the problem of module augmentation with method? : string

[Uzlopak]

Can you be more specific, please? What do you mean withmodule augmentation?

[plvo]

I was referring to this message when I was talking about the module augmentation

#4282 (comment)