链接承诺与以前的结果

Question

I'm trying to get my head around accessing previous results in promises as outlined here How do I access previous promise results in a .then() chain?

I've managed to get this working with 2 promises, but when I add a 3rd that makes a POST request to an API the promise result is my post request and not the response from that post request.

Basically the flow of my app is. I insert an item into a DB. Using that insertId I then insert multiple more items into a database that are 'children' of the first insertId. But I also need to then send these values to an API. This API will then return me another ID which I will ultimately associate with my own previous insertID. The code is roughly as follows (I've removed some other logic and error handling for the sake of brevity).

let name = req.body.name;
let value = req.body.values;
let obj = {name:name};
let entries = [];
let a = db.items.insertItem(name);

let b = a.then((data) => {
    let insertId = data.insertId;
    let promises = [];
    values.forEach((val) => {
           entries.push({value:val)})
           promises.push( db.values.insertValuesForItem(insertId,val));
    })
    obj.entries = entries; 
    return promises;

})
let c =  b.then((data) => {
     return request.post(constants.devUrl,
    {
        headers:{
        'Authorization': 'bearer ' + constants.developerToken,
        'content-type': 'application/json'
        },
        json:obj
    });
});

Promise.all([a,b,c]).then(([resa,resb,resc]) => {
   //resc here contains the post request info e.g the post data and headers
   // what I really want from resc is the response from my post request
    res.redirect(200,'/items/' +  resa.insertId);
})

As I mentioned previously, in resc I actually need the response from the API request, not details of the request itself. Any ideas how I achieve that?

Answer 1

Now that the question was reopened, reiterating what I said in the comments:

You have two problems in your code:

1. Incorrectly chaining the promises:

    let b = a.then((data) => { let insertId = data.insertId; let promises = []; values.forEach((val) => { entries.push({value:val)}) promises.push( db.values.insertValuesForItem(insertId,val)); }) obj.entries = entries; return promises; // this variable is accessed later as `data`, but // never actually used => it's pointless }) 

   Since you return promises , you are in fact returning a Promise that, only when fulfilled , will return your array of promises. This means that:

   1. inside the b success callback, you are getting data (which is promises array) but not doing anything with it.
   2. inside Promise.all([a,b,c]).then(([resa,resb,resc]) , resb is the array of promises, not the results of the db execution.

   I understand you're trying to synchronize the execution, but it's not the right approach.

2. Incorrectly expecting request.post to return a promise:

    let c = b.then((data) => { return request.post(constants.devUrl, { headers:{ 'Authorization': 'bearer ' + constants.developerToken, 'content-type': 'application/json' }, json:obj }); }); 

   As you mentioned in the comments, you're using this requests lib . As such, c in this case, is indeed a Promise , but its return value is not httpResponse as you expect.

   • If you want to work with requests so that it returns promises, you need to use one of these variances of the requests lib , which are returning promises.

   • If, however, you don't want to change the lib, you can synchornize the execution by also passing a callback function for the post method. For example

      var gotResponseCallBack = function(err, httpResponse, body){ return httpResponse; } request.post(constants.devUrl, { headers:{ 'Authorization': 'bearer ' + constants.developerToken, 'content-type': 'application/json' }, json:obj }, gotResponseCallBack);