Get result of a callback in a variable

Question

I'm in front of a huge problem that I cannot solve. I want to get the result of the function collection.find() of Mongodb then put the result in a variable that I could reuse in another function running independently. Because it's a little thorough to explain, here's the code:

client.connect(function (err) {
    assert.equal(null, err);
    const db = client.db(myDatabase);
    const collection = db.collection(myCollection);
    collection.find({}).toArray(function (err, docs) {
        //docs is the final result that I want to store in a variable
    });
});

$(myInput).change(function() {
    //using docs
})

'docs' being the result of a callback, I don't know how to retrieve it in a variable. I tried to store the whole thing in a variable, I tried global variables but nothing works, I still get undefined each time I run my program. So yes, I could run my function into the callback of collection.find()

client.connect(function (err) {
        assert.equal(null, err);
        const db = client.db(myDatabase);
        const collection = db.collection(myCollection);
        collection.find({}).toArray(function (err, docs) {
            $(myInput).change(function(docs) {
                //using docs
            })
        });
});

but since it's a function which I run a lot of time, it would call Mongo very often and that's not the best thing for performance especially since my database is running on another computer.

It's async, hence you need to call `myFunction` **inside** the .toArray callback, and pass `docs` as a function parameter (or make it global, but that wouldn't make much sense). Can you please share where myFunction is called? — briosheje, Oct 20 '18 at 16:37
I got clumsy when I write my question. myFunction is running independently and do things with docs. — palmtreesnative, Oct 20 '18 at 16:57
Can you please share the full code then? **where is myFunctions called?** — briosheje, Oct 20 '18 at 17:01
Possible duplicate of [How do I return the response from an asynchronous call?](https://stackoverflow.com/questions/14220321/how-do-i-return-the-response-from-an-asynchronous-call) — ghybs, Oct 20 '18 at 17:07

score 2 · Accepted Answer · answered Oct 20 '18 at 16:45

2

Define docs in the global scope and assign the data after the find method

let docs;

client.connect(async err => {
  assert.equal(null, err);
  const db = client.db(myDatabase);
  const collection = db.collection(myCollection);
  try {
    // docs is now a global variable containing all of the db collection
    docs = await collection.find({});
    myFunction();
  } catch (error) {
    // do something with the error
  }
});

function myFunction(){
  console.log("print docs", docs)
}

answered Oct 20 '18 at 16:45

Asaf Aviv

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myFunction is running independently which I forgot to mention. Put it in the callback could be a solution but I'm trying to do it differently (store docs in a global variable that could be reused independently in myFunction) – palmtreesnative Oct 20 '18 at 17:00
Do you wanna call `myFunction` from other files ? – Asaf Aviv Oct 20 '18 at 17:04
No, only this one (I'm calling these from a script other than main.js) – palmtreesnative Oct 20 '18 at 17:06
Why is my solution dosen't work for you ? docs is now a global variable containing the `collection.find` data which you can do whatever you want with it.. i think you should explain what you are trying to do thats failing – Asaf Aviv Oct 20 '18 at 17:10
I finally got your solution to work (mistake from me). Thank you a lot. I just wanted not to run myFunction in the callback but since the variable is now global I can use it independently. – palmtreesnative Oct 20 '18 at 17:17
Nice, glad to help. – Asaf Aviv Oct 20 '18 at 17:19

score 0 · Answer 2 · answered Oct 20 '18 at 16:57

Mongo already uses promises for async too (assuming v3+)

Let doc=[];

client.connect(function (err) {
assert.equal(null, err);
const db = client.db(myDatabase);
Let collection=               db.collection(myCollection);
collection.find({})
.then(results=>{
  //modify data
  doc.push(results);
  myfunction();
 }

});

 function  myFunction(){
console.log(doc)
  }

Get result of a callback in a variable

2 Answers2