feat(context): Custom header name for c.header

[akku1139]

Power UP #3221 .
You can define your own headers and the editor (e.g. VSCode) can complete them.

import { Hono } from "hono"

const app = new Hono<{
  Headers: "X-Hono-Test" | "Hello-Hono"
}>()

app.get("/", (c) => {
  c.header("Hello-Hono") // Defined header will be completed
  return c.text("Hello Hono!", 100, {
  })
})

The author should do the following, if applicable

• Add tests
• Run tests
• bun run format:fix && bun run lint:fix to format the code
• Add TSDoc/JSDoc to document the code

[yusukebe]

Hi @akku1139

Supporting adding extra header names is good. But I don't agree with using the generics for Hono. The generics for Hono have two types, Bindings and Variables. These are the most important types in the Hono type system. To keep the generics type definition simple, I think we don't have to add the Headers type for the generics. It should have only two important types.

I think it would be good to extend types in the following way:

https://hono.dev/docs/guides/jsx#override-type-definitions

[yusukebe]

Regarding adding custom header types, I think it's not good to pass generics to new Hono because of the above reason. Instead of it, we can extend the types with the following method if we can implement it:

declare module 'hono/context' {
  interface CustomResponseHeader {
    'X-Custom-Foo': 'foo' | 'foo2' | 'foo3'
    'X-Custom-Bar': 'bar' | 'bar2' | 'bar3'
  }
}

@nakasyou What do you think of it?

[nakasyou]

I think it is better than creating a generic.
Also it's more convenient if there are multi instances of Hono. I recommend the idea.