A simple TypeScript idea for an event dispatcher

eventer.ts

type Function = (data?: any) => void

const eventMap: Map<string, Function[]> = new Map()

function on(name: string, handler: Function) {
  const handlers = eventMap.get(name) || []
  handlers.push(handler)
  eventMap.set(name, handlers)
}

function off(name: string, handler?: Function) {
  if (handler) {
    const handlers = (eventMap.get(name) || []).filter((h) => h === handler)
    if (handlers.length) {
      eventMap.set(name, handlers)
    } else {
      eventMap.delete(name)
    }
  } else {
    eventMap.delete(name)
  }
}

function trigger<T = any>(name: string, data?: T) {
  const handlers = eventMap.get(name) || []
  for (const handler of handlers) {
    handler(data)
  }
}

export const eventer = { on, off, trigger }
export type Eventer = typeof eventer