Mitt.js

export default function mitt(all = new Map()) {
  return {
    /**
     * A Map of event names to registered handler functions.
     */
    all,

    /**
     * Register an event handler for the given type.
     * @param {string|symbol} type Type of event to listen for, or `"*"` for all events
     * @param {Function} handler Function to call in response to given event
     * @memberOf mitt
     */
    on(type, handler) {
      const handlers = all.get(type)
      const added = handlers && handlers.push(handler)
      if (!added) {
        all.set(type, [handler])
      }
    },

    /**
     * Remove an event handler for the given type.
     * @param {string|symbol} type Type of event to unregister `handler` from, or `"*"`
     * @param {Function} handler Handler function to remove
     * @memberOf mitt
     */
    off(type, handler) {
      const handlers = all.get(type)
      if (handlers) {
        handlers.splice(handlers.indexOf(handler) >>> 0, 1)
      }
    },

    /**
     * Invoke all handlers for the given type.
     * If present, `"*"` handlers are invoked after type-matched handlers.
     *
     * Note: Manually firing "*" handlers is not supported.
     *
     * @param {string|symbol} type The event type to invoke
     * @param {Any} [evt] Any value (object is recommended and powerful), passed to each handler
     * @memberOf mitt
     */
    emit(type, evt) {
      ;(all.get(type) ?? []).slice().map((handler) => {
        handler(evt)
      })
      ;(all.get('*') ?? []).slice().map((handler) => {
        handler(type, evt)
      })
    },
  }
}