异步/等待未按预期顺序执行

Question

I am currently attempting to learn about MongoDB and asynchronous functions in javascript and have been messing around for the past few hours with no luck.

async function getObj(name) {
  var db = await mongo.connect(DBURL, {
    useNewUrlParser: true
  });

  var dbo = db.db("testdb");

  var meme;

  await dbo.collection("files").findOne({
    "_id": name
  }, function(err, res) {
    console.log(res + " a");
    meme = res;
    console.log(meme + " b");
  });

  console.log(meme + " c");
  await db.close();
  console.log(meme + " d");
  return meme;
}

The program prints it in the order: cdab

The query works fine where if I do console.log(res), I get the object with the right results so that isn't the problem.

Why does the program ignore the await that is before it and end up printing and returning undefined?

Answer 1

You are not using Promises that you can await when you use db.findOne with a callback. You should await it like so:

 async function getObj(name) { var db = await mongo.connect(DBURL, { useNewUrlParser: true }); var dbo = db.db("testdb"); var meme = await dbo.collection("files").findOne({ "_id": name }); console.log(meme + " c"); await db.close(); console.log(meme + " d"); return meme; }