javascript Promise not wait Promise.all-CodePudding

So getAstronautsData make request to API then return array of promises. This promises mast make request to Wiki API and parse response in object. Then exampleAsyncFunc must wait all promises and return one big object with all info about Astronauts. But if I use Promise.all function ending and console is clear.

function getAstronautsData() {
    return new Promise((resolve, reject) => {
        getData('http://api.open-notify.org/astros.json', "http", (data) => {
            resolve(data) // get Astronauts list from API
        })
    }).then((astronautsList) => {
        return astronautsList.people.map((person => // return array of promises 
            new Promise(resolve => {
                getWikiData(person.name, (data) => { // request on Wiki API
                    resolve({info: data.extract, img: data.thumbnail.source})
                })
            })
        ))
    })
}


async function exampleAsyncFunc (){
    let promisesList = await getAstronautsData()
    // next code just few variant was i try
    let data = await Promise.all(promisesList)// it's not working.
    console.log(data) 

    Promise.all(promisesList).then(data => console.log(data)) //it's not working. Function display nothing

    
    promisesList.forEach((promise) =>  { //it's working but not so elegant
        promise.then(data => console.log(data))
    })

}

exampleAsyncFunc ()

CodePudding user response：

You appear to be using Promise.all correctly, but if any of the Promises in Promise.all rejects, then overall Promise.all promise will reject too and nothing will happen, where in your forEach version it'll simply skip those promises silently and move on to the next entries.

Likewise if any of the promises in the list stays pending: if so then the Promise.all promise will never resolve. This could be because you have a long list of return values and the whole list takes a longer-than-expected time to resolve, or because your getWikiData call encounters an error and you don't pass that out to reject that particular promise in your array.

You can debug this behavior by ensuring that each of your calls to then is followed by .catch(console.error) (or some more robust error handler).

CodePudding user response：

Let me first disclose that I am a big promise partisan and frankly deplore callbacks. The implication here is that I would not have written your getData and getWikiData with callbacks.

I will also point out that I second what @t.niese said in the comments: Because it does not make sense having both let data = await Promise.all(promisesList) and promisesList.forEach((promise) => {.

Anyway, your code is unnecessarily complex and can be simplified like so:

function getAstronautsData(callback) {
    getData('http://api.open-notify.org/astros.json', "http", data => {
        callback(data.people.map(person =>
            new Promise(resolve => {
                getWikiData(person.name, data => {
                    resolve(data);
                })
            }))
        )
    })
}

function exampleAsyncFunc (){
    getAstronautsData(promises => {
        Promise.all(promises)
            .then(result => {
                //result will contain those resolved promises
                console.log(result);
            })
    });
}

exampleAsyncFunc ()

Notice that I am passing a callback to getAstronautsData and call it from inside that function with the array of promises you ultimately want to resolve. No need for async here either as you can see.