javascript - 为什么 axios.get 没有返回 promise

Question

在下面的代码中，为什么最后一个 axios.get( url.toString()) 没有返回 Promise 而是返回 undefined ？我们该如何处理这个问题？

function getCommentersEmailIds( username ) {
    let foundUserId = undefined;
    //first get user id of username
    axios.get( 'https://jsonplaceholder.typicode.com/users?username='+username )
    .then(function( response ) {
        var foundUser = response.data.find(u=>u.username === username);
        return foundUser.id;
    })
    .then(function( userId ) {
        let url = 'https://jsonplaceholder.typicode.com/users/'+ userId.toString() +'/posts';
        return axios.get( url.toString());
    })
    .then(function( posts ) {
        //get id of the first post
        let postId = posts.data[0].id;
        let url = 'https://jsonplaceholder.typicode.com/comments?' + postId.toString();
        return axios.get(  url.toString());
    })
    .catch(function( error ) {
        console.log( error.message );
    });
}

var promiseForComments = getCommentersEmailIds('Bret');
promiseForComments.then(function( comments ) {
   comments.data.array.forEach(comment => {
       console.log(comment.email);
   }); 
})
.catch(function( error ) {
    console.log( error.message );
});

Answer 1

这里的问题是您缺少 return getCommentersEmailIds()中的声明而不是第二次调用 axios.get 所特有的问题:

function getCommentersEmailIds( username ) {
    let foundUserId = undefined;

    //Update: add return before axio.get
    return axios.get( 'https://jsonplaceholder.typicode.com/users?username='+username )
    .then(function( response ) {
        var foundUser = response.data.find(u=>u.username === username);
        return foundUser.id;
    })
    .then(function( userId ) {
        let url = 'https://jsonplaceholder.typicode.com/users/'+ userId.toString() +'/posts';
        return axios.get( url.toString());
    })
    .then(function( posts ) {
        //get id of the first post
        let postId = posts.data[0].id;
        let url = 'https://jsonplaceholder.typicode.com/comments?' + postId.toString();
        return axios.get(  url.toString());
    })
    .catch(function( error ) {
        console.log( error.message );
    });
}

通过返回 axios.get() 的 promise 来自getCommentersEmailIds() ，后续的链接(导致 forEach() 迭代到 comments.data.array )应该按预期执行。

希望有帮助!