Why can't I rename this object in my node application? - node.js

Question

I've got a node.js/express application communicating with a Postgres database using the node-postgres module. It's working with async/await, but if I rename the object being returned to anything other than { rows }, it comes back undefined. Here is my code; note the comment right under the async, and above const { rows } = ...:
var express = require('express');
var router = express.Router();
const { Pool } = require('pg');
const pool = new Pool({
connectionString: 'postgresql://postgres#localhost/mydb'
});
router.post('/login', function(req, res, next) {
var username = req.body.username;
var password = req.body.password;
(async () => {
// if I rename this from "rows" to, say, "userdetails", it comes back undefined
const { rows } = await pool.query(`
SELECT id, password
FROM myschema.users
WHERE username = $1
LIMIT 1;`,[username]);
if (rows.length) {
// check password, etc, etc.
return res.status(200).send();
} else {
return res.status(401).send();
}
})().catch(e => setImmediate(() => {
res.status(500);
}
));
});
module.exports = router;
I'm sure I'm missing something pretty basic here, but why can't I rename this? Does the pg module dictate somehow that the returned var/const must be named { rows }? If so, how might I discover that? I set a breakpoint at the await and step through the code, but it's still unclear to me.

Answer 1

When you do const { rows } = xxx, you are using object destructing to assign the xxx.rows property to a new variable in the local scope that has the same name as the property.
Thus when you change the name of that property to something else:
const { myVar } = xxx;
The code is now looking for the property xxx.myVar which likely does not exist and thus ends up undefined.
If you want to use a different name, then you have to either use a different form of object destructuring assignment (that includes a new name) or don't use destructuring at all and just assign the .rows property to a new variable.
For example, you could do this instead:
const result = await pool.query(`
SELECT id, password
FROM myschema.users
WHERE username = $1
LIMIT 1;`,[username]);
const myVar = result.rows;
Or you could designate a new name in the object destructuring assignment:
const {rows: myVar} = await pool.query(`
SELECT id, password
FROM myschema.users
WHERE username = $1
LIMIT 1;`,[username]);
console.log(myVar);

Answer 2

This is called Object destructuring. When you do something like this
const { row } = someObject
then the variable row is equal to the row property of someObject. But if someObject does not even have any row property, then row obviously will be undefined.
Same is happening in your case. The object returned by await pool.query(... have rows property, thats why const { rows } works but it does not have userdetails property hence undefined.