推送在节点 js 中的 map function 中不起作用

Question

I m using Mongo DB and after getting result from aggregate query, map the result of that query and try to push result to array. While consoling that array it returning an empty array, but if we console inside the map there are data s. Below is my code snippet

let house = await House.aggregate([
    { $group: { _id: "$houseNumber", count: { $sum: 1 } } },
    { $match: { _id: { $ne: null }, count: { $gt: 1 } } },
    { $project: { houseNumber: "$_id", _id: 0 } },
  ]);

  var arr = [];
  house.map(async (hNo) => {
    var house2 = await House.find(
      { houseNumber: hNo.houseNumber },
      "houseNumber"
    );
    arr.push(house2);
  });
  
  console.log(arr);

I m thinking async await making problem here. Please help me. Thank you so much.

Answer 1

Async function are not supported in.map

You can use Promise.all to await every async function result.

let house = await House.aggregate([
    { $group: { _id: "$houseNumber", count: { $sum: 1 } } },
    { $match: { _id: { $ne: null }, count: { $gt: 1 } } },
    { $project: { houseNumber: "$_id", _id: 0 } },
]);

// Array of Promises
const promises = house.map(hNo =>
    House.find(
        { houseNumber: hNo.houseNumber },
        "houseNumber"
    );
)

// Array with each promise result
const arr = await Promise.all(promises);
  
// Now you able to log it
console.log(arr);

While consoling that array it returning an empty array map is not waiting for an async function result, so at the moment of logging arr you have an empty array and arr.length pending Promises.