const xmlHttpRequest = new Promise((resolve, reject) => {
  setTimeout(() => {
      resolve('resolved')
    }, 1000) // 假装有异步
})

// `resolve`之后就`then`了
xmlHttpRequest.then(data => console.log(data))
console.log('sent')

const Request = i => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            console.log(i)
            resolve(i)
        }, 1000)    // 假装有异步
    })
}

// `resolve`之后就`then`
Request(i)
    .then(s => Request(2))
    .then(s => Request(3))
    .then(s => Request(4))