Q: Why doesn't Mitt support async event handlers? (from #122)

Asynchrony is expensive when unused, so it can't feasibly be landed in Mitt itself. Also, there isn't an obvious solution for how data should flow between handlers: a function being async implies it should have a return value (since async functions are just functions that return a Promise) - otherwise "async emit" is effectively "sequential emit" or a queue.

However you can implement it as an add-on. I'd recommend calling it emitAsync() or something, rather than overwriting the normal emit() method:

import mitt from 'mitt';

export function mittAsync(all) {
  const inst = mitt(all);
  inst.emitAsync = emitAsync;
  return inst;
}

async function emitAsync(type, e) {
  let handlers = this.all.get(type);
  if (handlers) for (const f of handlers) await f(e);
  let handlers = this.all.get('*');
  if (handlers) for (const f of handlers) await f(type, e);
}

// usage:
const events = mittWithAsync();
events.on('get', async url => fetch(url).then(r=>r.json()));
await events.emit('get', '/foo.json');