Promise review and summary (review the past and learn the new)

Promise review and summary (review the past and learn the new)

Reference: https://juejin.cn/post/6844903604009041928

1.promise syntax

PromiseThe core idea of ​​programming is that if the data is ready ( promised), then ( then) do something.

promise instance

const promise = new Promise(function(resolve, reject) {
    
    
  // ... some code

  if (/* 异步操作成功 */){
    
    
    resolve(value);
  } else {
    
    
    reject(error);
  }
})

PromiseThe constructor accepts a function as a parameter, and the two parameters of the function are resolveand reject.

resolve函数的作用是, change the state of the Promise object from “未完成”变为“成功”
pending to resolved, call it when the asynchronous operation is successful, and pass the result of the asynchronous operation as a parameter;

rejectThe function of the function is to change the state of the Promise object from “未完成”变为“失败”
pending to rejected, to be called when the asynchronous operation fails, and to pass the error reported by the asynchronous operation as a parameter.

After the Promise instance is generated, you can use the then method to specify the callback functions for the resolved state and rejected state respectively.

promise.then(function(value) {
    
    
  // success
}, function(error) {
    
    
  // failure
});

thenThe method can accept two callback functions as parameters.