Object is null when reading from JSON in NodeJs - json

Question

I got this server code running
const fs = require('fs');
const express = require('express');
const app = express();
app.get('/profile/:id', function (req, res) { // A route with a parameter
res.render('profile', {
user: getUserById(req.params.id)
});
});
app.listen(8888, function () {
console.log('Server running on port 8888');
});
function getUserById(userId){
fs.readFile('./database.json', 'utf8', function (err, data) {
var json = JSON.parse(data);
var users = json.users;
return users.find(u => u.id === userId);
});
}
And when calling the route, the function getUserById gets called. In my database, I have this data
{
"users": [
{
"id": 2312,
"name": "Foo Bar",
}
]
}
so the route would be /profile/2312 for example.
req.params.id returns the value 2312.
In the loop at var currentUser = users[0]; currentUser.id will return 2312 and the parameter passed in is 2312.
But when assigning user = currentUser; the object user is null.
Do I miss a module? Is the code wrong?

Answer 1

user object is null because you are returning it before your code actually reads the file.
fs.readFile('./database.json', 'utf8', function (err, data) { }
fs.readFile is asynchronous, so in order to return correct value you have to move the return statement inside fs.readFile block.
Also since getUserById is calling an asynchronous function, you have to call res.render after 'getuserById' finishes executing.
const fs = require('fs');
const express = require('express');
const app = express();
app.get('/profile/:id', getUserById);
app.listen(8888, function () {
console.log('Server running on port 8888');
});
function getUserById(req,res){ // Get a user from the database by userId
const userId = req.params.id;
fs.readFile('./database.json', 'utf8', function (err, data) {
var json = JSON.parse(data); // get the JSON object
var users = json.users; // convert the object to a user array
var match = users.find(u=>u.id.toString()===userId.toString());
//Call render after the asynchronous code finishes execution.
res.render('profile', {
user: match
});
});
}
How does Asynchronous Javascript Execution happen? and when not to use return statement?