index.ts

type EventName = string
type EventFnArgs = any[]
type EmitterContract = Record<EventName, EventFnArgs>

/**
 * Event Emitter that takes the expected contract as a generic
 * @example
 * ```ts
 *  type Contract = {
 *    delivery_success: [DeliverySuccessResponse, Metrics],
 *    delivery_failure: [DeliveryError]
 * }
 *  new Emitter<Contract>()
 *  .on('delivery_success', (res, metrics) => ...)
 *  .on('delivery_failure', (err) => ...)
 * ```
 */
export class Emitter<Contract extends EmitterContract = EmitterContract> {
  private callbacks: Partial<Contract> = {}
  on<EventName extends keyof Contract>(
    event: EventName,
    callback: (...args: Contract[EventName]) => void
  ): this {
    if (!this.callbacks[event]) {
      this.callbacks[event] = [callback] as Contract[EventName]
    } else {
      this.callbacks[event]!.push(callback)
    }
    return this
  }

  once<EventName extends keyof Contract>(
    event: EventName,
    callback: (...args: Contract[EventName]) => void
  ): this {
    const on = (...args: Contract[EventName]): void => {
      this.off(event, on)
      callback.apply(this, args)
    }

    this.on(event, on)
    return this
  }

  off<EventName extends keyof Contract>(
    event: EventName,
    callback: (...args: Contract[EventName]) => void
  ): this {
    const fns = this.callbacks[event] ?? []
    const without = fns.filter((fn) => fn !== callback) as Contract[EventName]
    this.callbacks[event] = without
    return this
  }

  emit<EventName extends keyof Contract>(
    event: EventName,
    ...args: Contract[EventName]
  ): this {
    const callbacks = this.callbacks[event] ?? []
    callbacks.forEach((callback) => {
      callback.apply(this, args)
    })
    return this
  }
}