異步加載 JS
ES5

function loadScript (src, callback) {
  let script = document.createElement('script')
  script.src = src
  script.onload = () => { callback(src) }
  script.onerr = (err) => { callback(err) }
  document.head.append(script)
}

function test (name) {
  console.log(name)
}

loadScript('./1.js', function (script) {
  if (script.message) {
    // 監(jiān)控上報邏輯
    loadScript('./2.js', function (script) {
      console.log(script)
    })
  } else {
    console.log(script)
    loadScript('./2.js', function (script) {
      console.log(script)
      loadScript('./3.js', function (script) {
        console.log(script)
      })
    })
  }
})

ES6 promise

function loadScript (src) {
  // pending,undefined
  return new Promise((resolve, reject) => {
    let script = document.createElement('script')
    script.src = src
    script.onload = () => resolve(src)// fulfilled,result
    script.onerror = (err) => reject(err)// rejected,error
    document.head.append(script)
  })
}

loadScript('./1.js')
  .then(() => {
    return loadScript('./4.js')
  }, (err) => {
    console.log(err)
  })
  .then(() => {
    loadScript('./3.js')
  }, (err) => {
    console.log(err)
  })


// promise.then(onFulfilled,onRejected)

function test (bool) {
  if (bool) {
    return new Promise((resolve, reject) => {
      resolve(30)
    })
  } else {
    return Promise.reject(new Error('ss'))
  }
}
test(0).then((value) => {
  console.log(value)
}, (err) => {
  console.log(err)
})
function loadScript (src) {
  // pending,undefined
  return new Promise((resolve, reject) => {
    let script = document.createElement('script')
    script.src = src
    script.onload = () => resolve(src)// fulfilled,result
    script.onerror = (err) => reject(err)// rejected,error
    document.head.append(script)
  })
}

loadScript('./1.js')
  .then(() => {
    return loadScript('./2.js')
  })
  .then(() => {
    return loadScript('./4.js')
  })
  .catch(err => {
    console.log(err)
  })
  // throw new Error
const p1 = Promise.resolve(1)
const p2 = Promise.resolve(20)
const p3 = Promise.resolve(3)
// 等待 p1, p2, p3 都執(zhí)行完成
Promise.all([p1, p2, p3]).then((value) => {
  console.log(value)
})

const p1 = () => {
  return new Promise((resolve, reject) => {
    setTimeout(function () {
      resolve(1)
    }, 1000)
  })
}

const p2 = () => {
  return new Promise((resolve, reject) => {
    setTimeout(function () {
      resolve(2)
    }, 3000)
  })
}
// p1, p2, p3 有一個執(zhí)行完成
Promise.race([p1(), p2()]).then((value) => {
  console.log(value)
})

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