Promise processing instance that completes multiple interdependent asynchronous requests

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In projects, we often encounter multiple asynchronous requests that depend on each other. If there are three ajax requests a, b, and c, b needs to rely on the data returned by a, and c needs the data returned by a and b requests. It is naturally not advisable to use nested requests. It makes the code difficult to maintain and there are many requests. Many questions will arise.

Promise is to solve the problem of multiple asynchronous requests. Promise is an object provided by ES6, used to deliver messages for asynchronous operations.

Promise has three states: Pending (in progress), Resolved (completed, also known as Fulfilled) and Rejected (failed).

Upload the code directly. There are requests for a and b, and b depends on the request data of a. As follows:

function a(){
      return new Promise(function(res,rej){
        $.ajax({
          url:"a",
          type: "GET",
          async:true,
          dataType:"json",
          success:function(data){
            console.log(data,"a");
            res(data);
          }
        })
      });
    }
    function b(data){
      console.log(data,"data");
      return new Promise(function(res,rej){
        $.ajax({
            url:"b",
            type: "POST",
            async:true,
            data:JSON.stringify(data),
            dataType:"json",
            success:function(data){
              console.log(data,"b");
              res();
            }
          })
      });
    }
    $("#btn").click(function(){
      a().then(function (data){
        b(data);
      }).then(function(){
      })
    })

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