Zero Runtime Factory

[miyaji255]

What is the feature you are proposing?

Description

Since it is common for projects to use a single Env type, I propose using declaration merging as an alternative approach.

I believe this change can simplify type management and improve the developer experience, and this approach can also support presets.

Proposed Method

To address this, I propose modifying the Hono class as follows:

- class Hono<E extends Env = Env, ...>
+ class Hono<E extends Env = DefaultEnv, ...>

+ interface DefaultEnv { }
+ interface EnvBindings { }
+ interface EnvVariables { }

This allows developers to define and extend DefaultEnv for their projects without needing to specify Env types in every instance:

declare module "hono" {
  interface DefaultEnv {
    Bindings: EnvBindings;
    Variables: EnvVariables;
  }
  interface EnvBindings {
    lambdaContext: LambdaContext;
  }
  interface EnvVariables {
    foo: string;
  }
}

const app = new Hono();
const middleware: MiddlewareHandler = (c, next) => ...

Example for AWS Lambda

For example, adapters could automatically set Bindings when using AWS Lambda:

// aws-lambda/handler.ts
declare module "hono" {
  interface DefaultEnv {
    Bindings: EnvBindings
  }
  interface EnvBindings {
    event: LambdaEvent
    lambdaContext: LambdaContext
  }
}
export const handler ...