returning sqlite3 query in Node - sql

Question

I'm trying to write a function that returns a query from a sqlite3 database (using Node and Express)
This is how (likely) the function is called
app.get('/example',(req,res)=>{
console.log(getThings(db_connection))
}
And this is the function per se
getThings(db){
let sql = 'SELECT * FROM Table'
let results[]
db.all(sql, (err, rows) => {
if(err){throw err}
let i
for(i=0;i<rows.length;i++){
res.push(rows[i])
}
console.log(res)
})
return res
}
I expected the rows being returned at the end, but it always returns res before populating it first, and just then it prints res with the correctly
I might have understood why it does so, but I have no idea how to fix it properly (I'm still new at JS)

Answer 1

Callbacks are asynchronous, so res will not be populated before the return.
You need to make your callback into a Promise or use async/await.
Promisify the callback:
getThings(db){
return new Promise((resolve, reject) => {
db.all('SELECT * FROM Table', (err, rows) => {
if (err) reject(err)
resolve(rows)
})
})
}
app.get('/example', (req, res) => {
getThings(db_connection)
.then(rows => res.send(result))
.catch(err => res.send({
error: err.message
}))
}
or
Use async/await:
Wrapped in try/catch to catch err, then because you're simply looping over the rows you don't need the for loop.
When you see the following structure object.method('param', (err, rows) => { you can almost guarantee its Promise compatible, else if not you can use util.promisify(original) to make it so.
Cleaner and more verbose then then().catch() chaining:
app.get('/example', async (req, res) => {
let result = {}
try {
result = await getThings(db_connection)
} catch (err) {
result = {
error: err.message
}
}
res.send(result)
}
async getThings(db) {
return db.all('SELECT * FROM Table')
}