property restriction: pending property

[loynoir]

Suggestion

🔍 Search Terms

• property restriction
• koa middleware

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

// Error: Property 'foo' is not set once.
async function koaMiddleware1(
  ctx: {pending foo: 1}
) {}

// OK
async function koaMiddleware2(
  ctx: {pending foo: 1}
) {
  ctx.foo = 1
}

// OK
async function koaMiddleware3(
  ctx: {pending foo: 1}
) {
  ctx.foo = ctx.foo ?? 1
}

Calling function

1. Maybe like below?

// OK
await koaMiddleware2({})

// OK
await koaMiddleware2({foo: 1})

2. Or maybe cast koaMiddlewareImplementX to another function koaMiddlewareX with type with ctx: {foo: 1}

📃 Motivating Example

Describe restriction on a koa middleware, should always set specific properties.

💻 Use Cases