1. ES6 grammar
  2. Promise is a solution to asynchronous programming
  3. new Promise((resolve, reject) => {…… })

  • When do you use Promise

    Typically, when there is an asynchronous operation, use Promise to encapsulate the asynchronous operation

    // The Promise argument is a function, here (resolve, reject) => {}
// This function takes two arguments: resolve,reject, and reject
// chain programming
new Promise((resolve, reject) => {
  // Asynchronous operation
  setTimeout((data) => {
    // Judge success or failure here
  	// Call resolve on success, then.then()
  	if(){
      resolve(data);
    } else {
       // Call reject on failure
    	reject('error.. ')}},1000)
  }).then((data) => {
  	// Where the business is actually done
  	console.log("hello world");
}).catch(err => {
  console.log(err);
})
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  • Three states of Promise

    • Pending: Pending state, such as network request in progress or timer out of time
    • Fulfill: fulfill state (fulfill state) Fulfill state (fulfill state) Fulfill state (fulfill state)
    • Reject: Reject state, which is in when we actively call back, and is called back. Catch ()

  • The chain call to Promise

    • In.then() there is no asynchronous operation, but the data from the previous step needs to be processed. For logical layering, chain layering is possible

      One of two ways

      1. Using Promise. Resovle (newdata) :return Promise.resolve(data + '2222')
      2. Return newdata:return data+'111'
      / / a layer
.then((data) => {
        console.log(data);
        // The next layer of res comes from here
        return Promise.resolve(data + '2222')
        //return data+'2222'
    }).then(res => {
        console.log(res);
    })
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    • In. Then, no asynchronous operation is performed, but the err of the previous step needs to be processed. For logical layering, chain layering can be performed

    • One of two ways

      1. Using Promise. Reject (err) :return Promise.reject(err + '2222')
      2. Throw err: ‘throw data+’111’
      .then(res => {
        console.log(res);
        return Promise.reject(res + '+err1')
        //throw res + '111'
    }).catch(err => {
        console.log(err);
    })
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  • Promise.all()

    Promise.all([
  Promise1, 
  Promise2
]).then(res => {
  / / Promise1 results
  res[0]...
  / / Promise2 results
  res[1]...  
})
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