Why even after using async await for multiple calls still got empty response?

问题

Here is simple node route in which calling an asynchronous api.

What needed is to return data after the looping. But It is returning blank object.

try {
  const array = ["brunch", "lunch", "crunch"]
  const data = {}
  array.map(async(d) => {
    const venue = await Venue.find({ "category": { "$in": [d] }})
    data[d] = venue
  })
  return data
} catch(err) {
  throw err
}

Please help me to achieve this

回答1:

There is a better way to get the desired result with MongoDB and no need to loop, use the aggregation framework where you can run the following pipeline which uses $facet as

try {
    const array = ["brunch", "lunch", "crunch"]
    const facet = array.reduce((acc, cur) => {
        acc[cur] = [{ "$match": { "category": cur } }]
        return acc
    }, {})
    const pipeline = [
        { "$match": { "category": { "$in": array } } },
        { "$facet": facet }
    ]
    const results = await Venue.aggregate(pipeline).exec()
    const data = results[0]

    return data
} catch(err) {
    throw err
}

---

You can also group the documents by the category key and $push the documents per group and then convert into keys of a document in a $replaceRoot with $arrayToObject

try {
    const array = ["brunch", "lunch", "crunch"]
    const pipeline = [
        { "$match": { "category": { "$in": array } } },
        { "$group": { 
            "_id": "$category",
            "data": { "$push": "$$ROOT" }
        } },
        { "$group": {
            "_id": null,
            "venues": {
                "$push": {
                    "k": "$_id",
                    "v": "$data"
                }
            } 
        } },
        { "$replaceRoot": {
            "newRoot": { "$arrayToObject": "$venues" }
        } }
    ]
    const results = await Venue.aggregate(pipeline).exec()
    const data = results[0]

    return data
} catch(err) {
    throw err
}

回答2:

Although @chridam approach is quite unique and maybe more efficient, in case you want to stick with a loop.

There are two approaches. You want all your operation to be run in parallel or series.

If parallel, you will have to use Promise.all.

try {
  const array = ["brunch", "lunch", "crunch"]
  const data = {}
  await Promise.all(array.map(async(d) => {
    data[d] = await Venue.find({ "category": { "$in": [d] }})
  }))
  return data
} catch(err) {
  throw err
}

If series, you will have to use simple for loop.

array.map(async(d) => {}) is making the internal db call asynchronous and not waiting for operation. normal for loop will be synchronous.

try {
  const array = ["brunch", "lunch", "crunch"]
  const data = {}
  for (d of array) {
    data[d] = await Venue.find({ "category": { "$in": [d] }})
  }
  return data
} catch(err) {
  throw err
}

回答3:

Problem is the way you are using async-await. It's inside a Array.prototype.map which made .map function asynchronous and thus main thread never waits for loop to complete and move forward to next statement and returned data which is just {}.

try {
  const array = ["brunch", "lunch", "crunch"]
  const data = {};

  // here's the issue async is callback function to map, due to which never waited for map function to finished.
  array.map( async(d) => {
    const venue = await Venue.find({ "category": { "$in": [d] }})
    data[d] = venue;
  });

  return data;
} catch(err) {
  throw err
}

Change code to this:

(async function () {
    try {
      const array = ["brunch", "lunch", "crunch"]
      const data = {};

      array.map( d => {
        const venue = await Venue.find({ "category": { "$in": [d] }})
        data[d] = venue;
      });

      return data;

    }
    catch(err) {
      throw err
    }
})();

What you did is something similar to this

function main () {
    return new Promise ( resolve => {
        setTimeout( () => {
            console.log("First");
            resolve(true);
        },5000 );
    });
}

(async function () {
    await main();

    console.log("Second")
})();

console.log("Third");
// Third
// First
// Second

来源：https://stackoverflow.com/questions/52534679/why-even-after-using-async-await-for-multiple-calls-still-got-empty-response