jQuery ajax调用then函数

Question

So I need two ajax calls to get all the data. I am using jQuery 's ajax call to achieve that. But then I am a bit confused at the execution order. Here is my problematic code:

$.ajax({
type: "GET",
url: "/api/data",
dataType: "json"
}).then(function (data) {
   console.log("I am the first")//correct
}).then(function () {
   //second ajax
    $.ajax({
    type: "GET",
    url: "/api/lifecyclephase",
    dataType: "json"
    }).then(function (data) {
       console.log("I am the second")//third
    })
 }).then(function () {
     console.log("I am the third")//second
 })

The output is

I am the first
I am the third
I am the second

Should not the then wait for the second ajax to finish its job before proceeding?

The correct one:

$.ajax({
  type: "GET",
  url: "/api/data",
  dataType: "json"
}).then(function (data) {
  console.log("I am the first")
}).then(function () {
  $.ajax({
    type: "GET",
    url: "/api/lifecyclephase",
    dataType: "json"
  }).then(function () {
    console.log("I am the second")
  }).then(function(){
    console.log("I am the third")
  })
})

Answer 1

The "second" $.ajax is initialized within the second .then , but that $.ajax isn't chained with anything else - the interpreter initializes the request and that's it, so when the end of the second .then is reached, the next .then (the third ) executes immediately.

Try return ing the second Promise instead - a subsequent .then will only wait for a Promise to resolve if that Promise is returned by the previous .then :

.then(function (data) {
   console.log("I am the first")//correct
})
.then(function () {
  //second ajax
  return $.ajax({
  // ...

Answer 2

In the problematic code, you are simply missing a return .

$.ajax({
    type: "GET",
    url: "/api/data",
    dataType: "json"
}).then(function (data) {
    console.log("I am the first");
}).then(function () {
    return $.ajax({
    ^^^^^^
        type: "GET",
        url: "/api/lifecyclephase",
        dataType: "json"
    }).then(function (data) {
        console.log("I am the second");
    });
}).then(function () {
    console.log("I am the third");
});

Without the return , there's nothing to inform the outer promise chain of the inner promise's exitence, hence the outer chain does not wait for the inner promise to settle before proceeding to the third stage.