How to assign the callback variable "cursor.toArray (err, doc)" to an external variable?

The problem is just Sync

You are experiencing this error because the find () method is asynchronous and in this case you have a Promise that may not have been resolved before console.log(result) ,

var result = [];
var user = db.collection('user');
var cursor = user.find(); 

//Aqui você tem uma Promisse
cursor.toArray(function (err, doc) {
    result = doc;
});
console.log(result);//Caso a Promisse não tenha sido executada ainda, result ainda é um Array vazio

An interesting way to run could be like this, depending on your version of Node :

async function getResults() {
    db.collection('user').find()
}

var results = await getResults();
results = results.toArray();

Dai your code looks like a synchronous code. The key here is the async / await command that waits for the promise results. More information, link link

It looks like this:

function find(query, callback) {

    db.collection('user').find(query).toArray(function (err, doc) {
        callback(err, doc);
    });
}

find({name: 'Leandro'}, function (err, doc) {

    let result = [];
    result['user'] = doc;
    console.log(result['user']); // console -> [ { name: 'Leandro', _id: 5a1d624e79a88f09fb805a35 } ]
});