Is it possible to use axios.all with a then() for each promise?

Question

I have a React component that triggers an event to fetch data. This results in a dynamic number of stored proc calls to fetch data, and the data from each call is stored in a totally different location. Then I need to re-render once all of the data is received and available. I'm using promises with axios.

Since the number of axios calls is dynamic, I'm building an array and inserting it into axios.all as follows:

let promises = [];

for (let i = 0; i < requests.length; i++) {
    promises.push(axios.get(request[i].url, { params: {...} }));
}

axios.all(promises).then(/* use the data */);

The problem is that each axios request returns data that gets added to an object in a totally different place. Since I have no way to put them all in the correct place in a single then (how would I know which response goes in which location?), I tried doing something like this:

let promises = [];

for (let i = 0; i < requests.length; i++) {
    promises.push(
        axios.get(request[i].url, { params: {...} })
            .then(response => {myObject[request[i].saveLocation] = response.data;})
    );
}

axios.all(promises).then(/* use the data */);

However, this doesn't work as I expected. The then after each get is executed, but not until well after the then attached to axios.all. Obviously this is a problem because my code tries to use the data before it has been saved to the object.

Is there a way to have a separate then call for each axios.get that will be executed after its corresponding promise is resolved, and then have a final then that will be executed only after all of the promises are resolved, to use the data now that the object has been populated?

Answer 1

Okay, so I found a way to do what I needed without using using a then on each get. Since the params passed in to axios.get contain enough info to determine the save location, and since I can read the params back from the response, I can do something like the following:

let promises = [];

for (let i = 0; i < requests.length; i++) {
    promises.push(axios.get(request[i].url, { params: {...} }));
}

axios.all(promises)
    .then(axios.spread((...args) => {
        for (let i = 0; i < args.length; i++) {
            myObject[args[i].config.params.saveLocation] = args[i].data;
        }
    }))
    .then(/* use the data */);

This ensures all the data is received and saved to the object before it is used.

Answer 2

If the behaviour of your second attempt is indeed like that, then that would be an indication that axios is not Promise/A+ compliant. The then callback's return value must be the value with which the promise returned by that then is fulfilled. Since that is the promise you push into the array, the value that axios.all would return for that promise can only be known by executing the then callbacks first.

Event though you do not return a value explicitly in the then callback, this does not affect the above rule: in that case the return value is undefined and it is that value that should be provided by axios.all once the corresponding promise is resolved.

See in particular the rules 2.2.7, 2.2.7.1, 2.3.2.1, 2.3.2.2 in the specs of Promise/A+).

So I would suggest using a Promise/A+ compliant promise implementation instead. There are several other libraries, like for instance request-promise.

Alternatively, you could use the native ES6 Promise implementation, and promisify the http.request method yourself.

ES6 offers Promise.all which guarantees to provide the resolved values in the same order as the promises were provided.

Answer 3

your initial code could work normally as intended if you pass the promises to your array attached with their respective then function

let promises = []; // array to hold all requests promises with their then
for (let i = 0; i < requests.length; i++) {
    // adding every request to the array
    promises.push(
        axios.get(request[i].url, { params: { ...} })
            .then(response => { myObject[request[i].saveLocation] = response.data; })
    );
}
// Resolving requests with their callbacks before procedding to the last then callback
axios.all(promises).then(/* use the data */);