About the local variable in Node.js promise - node.js

Question

I am a newer of Node.js.I defined a array as a local variable,and want to use it in the following then,I save some useful data in it.But in the end, the array is empty.Can somebody tell me why?Thanks for your support.
const Device = require("./mongo.js").Device;
const Video = require("./mongo.js").Video;
Device.findOne({id:"11112222"}).exec()
.then(function(data){
var videoIds = data.videoIds.split(",");
var videoId2URL = [];
console.log(videoIds);
videoIds.forEach(function(one){
return Video.findOne({id:one}).exec()
.then(function(data){
videoId2URL.push({id:one,url:data.url});
return videoId2URL;
})
});
console.log(videoId2URL);
});

Answer 1

The problem is that you are displaying videoId2URL too early.
Device.findOne returns a promise executed asynchronously. But Video.findOne also returns a promise executed asynchronously.
So when you do console.log(videoId2URL);, the promises created by Video.findOne are not executed yet. So your array is empty.
You must wait the end of all your promises. You can use Promise.all for that.
Promise.all(videoIds.map(function(one){
return Video.findOne({id:one}).exec()
.then(function(data){
videoId2URL.push({id:one,url:data.url});
return videoId2URL;
});
})
.then(function() {
console.log(videoId2URL);
});

Answer 2

You could use Promise.all to resolve your problem. You forEach code contains async code. Your last line does not wait for all promises to get resolved.
Try with:
var arr = [];
videoIds.forEach(function(one){
return arr.push(Video.findOne({id:one}).exec());
});
Promise.all(arr) // here we are waiting for all async tasks to get resolved
.then(function(data){
console.log(data);
// parse your data here and find array of videoId2URL
})

Answer 3

When you do console.log(videoId2URL), you're still in the main stack for the script, while none of the push callbacks have been executed.
You can use an array to collect the promises returned by Video.findOne, and at the end use Promise.all to drain all the promises and do the log then.
BTW, none of the 2 return are necessary, you can safely remove them.
The 1st one is not used because it's in a synchronous callback for forEach.
The 2nd one is not used because you're relying on the side effect, rather than use the resolved value.
Try:
const Device = require("./mongo.js").Device;
const Video = require("./mongo.js").Video;
Device.findOne({id:"11112222"}).exec()
.then(function(data){
var videoIds = data.videoIds.split(",");
var videoId2URL = [];
var promiseArr = [];
console.log(videoIds);
videoIds.forEach(function(one){
var p = Video.findOne({id:one}).exec()
.then(function(data){
videoId2URL.push({id:one,url:data.url});
});
promiseArr.push(p);
});
Promise.all(promiseArr).then(function() {
console.log(videoId2URL);
});
});