Thinking--函数同时支持 callback & promise

function fn (cb?: Function, ctx?: Object) {
  // 函数内部逻辑
  console.log('...')
  // promise
  if (!cb && typeof Promise !== 'undefined') {
    return new Promise(resolve => {
      resolve(ctx)
    })
  }
  // callback
  if (cb) {
  	cb.call(ctx)
  }
}

fn(() => console.log('callback')) 	// ... => callback

fn().then(() => {
	console.log('promise resolve')	// ... => promise resolve
})

(async function () {
  await fn()
  console.log('async...await')		// ... => async...await
})()