Strongly typed event names & values

[franky47]

I really like mitt, and the rewrite in TypeScript is a great step forward, but I wonder if we could go a bit further and allow strong typing of both the event names and their associated values.

Ideally, this would be an opt-in behaviour, meaning that by default mitt uses a string | symbol type for event names, and any for values. But passing an interface as a generic type to the constructor would tell mitt what events to expect and what values are associated, along with providing auto-completion in IDEs.

Hypothetical API:

interface MittEvents
{
  foo: number
  bar: string
  egg: {
    a: string[]
    b?: number
  }
  noarg: null
}

const emitter = mitt<MittEvents>()

emitter.on('foo', value => {
  // value has type `number`
})

emitter.on('bar', value => {
  // value has type `string`
})

emitter.on('egg', ({ a, b = 0 }) => {
  // a has type `string[]`
  // b has type `number` with a default value
})

emitter.emit('foo', 42)
emitter.emit('noarg', null)

// Check value type
emitter.emit('foo', 'not a number') // <- TS error

// Check allowed event names
emitter.emit('another', 42) // <- TS error

Such a typing system can be seen in action in vegemite, a state management library with a similar EventEmitter interface.

[developit]

@franky47 I like the idea! Curious how we could do this given the move to Map though.

[ZeroThe2nd]

Is this still being planned for implementation, or will this be dropped altogether in its current state?

[franky47]

PR #114 is exactly what I'm looking for, FYI.

[ZeroThe2nd]

@franky47 It'll need to be approved first though. It would be a nice addition for sure.

[danielweck]

This TypeScript userland solution (outside of Mitt) works perfectly fine:
#112 (comment)