在继续之前如何等待所有诺言得到解决

Question

I have this code

     async function concurrent() {
        await Promise.all([
            getColorCount('black'),
            getColorCount('purple'),
            getColorCount('red'),
            getColorCount('amber'),
            getColorCount('yellow'),
            getColorCount('white'),
            buildChartData()
        ]);

    }
    concurrent();
    res.json(chartData); //This is run before above promises are complete

My understanding was that async await halted the code until complete, but in this case, res.json(chartData) runs before the above promises have been completed.

How do I ensure that all promises have completed before responding to the client with res.json(chartData); ?

Answer 1

return a value from the function, or use .then() within concurrent() function call

 function concurrent() {
   return Promise.all([
        getColorCount('black'),
        getColorCount('purple'),
        getColorCount('red'),
        getColorCount('amber'),
        getColorCount('yellow'),
        getColorCount('white'),
        buildChartData()
    ]);

  }
  concurrent()
  .then(chartData => 
    res.json(chartData)
  )
  .catch(err => console.error(err));

 async function concurrent() {
   const promises = await Promise.all([
        getColorCount('black'),
        getColorCount('purple'),
        getColorCount('red'),
        getColorCount('amber'),
        getColorCount('yellow'),
        getColorCount('white'),
        buildChartData()
    ]);
    return promises;
  }
  concurrent()
  .then(chartData => 
    res.json(chartData)
  )
  .catch(err => console.error(err));

Answer 2

Use

concurrent().then(promiseResult => {
      res.json(chartData);
});

An async function always returns a promise and hence using "then", you can ensure the completion of an async function in a synchronised way.

Answer 3

You will need .then after promise.all. Then will return an array of values returned from your function calls in order they were initially called.

Promise.all([ getColorCount('black'), getColorCount('purple'), getColorCount('red'), getColorCount('amber'), getColorCount('yellow'), getColorCount('white'), buildChartData() ]) .then( function( responeses ) { console.log ( responses )};

And take a look at the responses array, it should all fall into place

Answer 4

concurrent is an asynchrounous function, as you indicate with the keyword async . That's why res.json is executed just after calling concurrent, without waiting.

You can be tempted to use await like this:

await concurrent();
res.json(chartData);

but it will throw an error because await can't be used outside a async context.

Unluckily you will need the traditional then to fulfill the promise:

 concurrent().then(() => {
     res.json(chartData);
 });

Apart from that, is buildChartData asynchronous? Maybe it should be executed outside Promimse.all