I am able to insert and retrieve data from an neDB database in nodejs. But I cannot pass the data outside of the function that retrieves it.
I have read through the neDB documentation and I searched for and tried different binations of callbacks and returns (see code below) without finding a solution.
I'm new to javascript so I do not know if I am misunderstanding how to use variables in general or if this issue is related to using neDB specifically or both.
Could someone please explain why "x" in my code does not ever contain the docs JSON results from the database? How can I make it work?
var fs = require('fs'),
Datastore = require('nedb')
, db = new Datastore({ filename: 'datastore', autoload: true });
//generate data to add to datafile
var document = { Shift: "Late"
, StartTime: "4:00PM"
, EndTime: "12:00AM"
};
// add the generated data to datafile
db.insert(document, function (err, newDoc) {
});
//test to ensure that this search returns data
db.find({ }, function (err, docs) {
console.log(JSON.stringify(docs)); // logs all of the data in docs
});
//attempt to get a variable "x" that has all
//of the data from the datafile
var x = function(err, callback){
db.find({ }, function (err, docs) {
callback(docs);
});
};
console.log(x); //logs "[Function]"
var x = db.find({ }, function (err, docs) {
return docs;
});
console.log(x); //logs "undefined"
var x = db.find({ }, function (err, docs) {
});
console.log(x); //logs "undefined"*
I am able to insert and retrieve data from an neDB database in nodejs. But I cannot pass the data outside of the function that retrieves it.
I have read through the neDB documentation and I searched for and tried different binations of callbacks and returns (see code below) without finding a solution.
I'm new to javascript so I do not know if I am misunderstanding how to use variables in general or if this issue is related to using neDB specifically or both.
Could someone please explain why "x" in my code does not ever contain the docs JSON results from the database? How can I make it work?
var fs = require('fs'),
Datastore = require('nedb')
, db = new Datastore({ filename: 'datastore', autoload: true });
//generate data to add to datafile
var document = { Shift: "Late"
, StartTime: "4:00PM"
, EndTime: "12:00AM"
};
// add the generated data to datafile
db.insert(document, function (err, newDoc) {
});
//test to ensure that this search returns data
db.find({ }, function (err, docs) {
console.log(JSON.stringify(docs)); // logs all of the data in docs
});
//attempt to get a variable "x" that has all
//of the data from the datafile
var x = function(err, callback){
db.find({ }, function (err, docs) {
callback(docs);
});
};
console.log(x); //logs "[Function]"
var x = db.find({ }, function (err, docs) {
return docs;
});
console.log(x); //logs "undefined"
var x = db.find({ }, function (err, docs) {
});
console.log(x); //logs "undefined"*
3 Answers
Reset to default 6Callbacks are generally asynchronous in JavaScript meaning you can not use assignment operator, consequently you do not return anything from the callback function.
When you call an async function execution of you programme carries on, passing the 'var x = whatever' statement. The assignment to a variable, the result of whatever callback is received, you need to perform from within the callback itself... what you need is something in the lines of ...
var x = null;
db.find({ }, function (err, docs) {
x = docs;
do_something_when_you_get_your_result();
});
function do_something_when_you_get_your_result() {
console.log(x); // x have docs now
}
EDIT
Here is a nice blog post about asynchronous programming. And there is a lot more resources on this topic that you can pick up.
This is a popular library to help with async flow-control for node.
P.S.
Hope this helps. Please, by all means ask if you need something clarified :)
I ran into the same problem. In the end I used a bination between async-await and a promise with resolve to solve it.
In your example the following would work:
var x = new Promise((resolve,reject) {
db.find({ }, function (err, docs) {
resolve(docs);
});
});
console.log(x);
I had to learn a bit about async functions to get it right. For those who are looking for specific help about getting a return value from nedb, here's a snippet of what worked for me. I was using it in electron.
function findUser(searchParams,callBackFn)
{
db.find({}, function (err, docs))
{
//executes the callback
callBackFn(docs)
};
}
usage
findUser('John Doe',/*this is the callback -> */function(users){
for(i = 0; i < users.length; i++){
//the data will be here now
//eg users.phone will display the user's phone number
}})