功能承諾

地圖示例：

Promise.resolve([ 1, 2, 3 ]).map(el => {
   return Promise.resolve(el * el) // return some async operation in real world
})

過濾器示例：

Promise.resolve([ 1, 2, 3 ]).filter(el => {
  return Promise.resolve(el % 2 === 0) // return some async operation in real world
}).then(console.log)

減少的例子：

Promise.resolve([ 1, 2, 3 ]).reduce((prev, curr) => {
  return Promise.resolve(prev + curr) // return some async operation in real world
}).then(console.log)