running queries in firebase using async / await

Question

Appreciating that firebase has added support for promises, is there a way to run a query like the following inside of an async function?:

const eventref = this.db.ref('cats/whiskers');
const value = await eventref.once('value')

Running the above returns a promise for value, I'm hoping to get the json blob that is stored at cats/whiskers.

Yes, why wouldn't it be possible? Have you tried it? Do you have a problem? — Bergi, May 01 '17 at 00:41
Hm, that should not be possible, `await` cannot return thenables. Do you use a transpiler? Have you inspected what the `eventref.once('value')` looks like in a debugger? — Bergi, May 01 '17 at 00:54
Works for me: https://runkit.com/5906914d3ff14f0012d0307e/590691ab301f4d0012938bc5 — Frank van Puffelen, May 01 '17 at 01:39
yeah, for some reason I was sticking the `.val()` call in the await statement, this worked for me, too. — fox, May 01 '17 at 03:49
`.once` works but `.on` (so `someRef.on('child_added')`) expects a second argument. Any ideas for that? — Karoh, May 06 '17 at 13:22

score 71 · Accepted Answer · answered Dec 08 '17 at 11:29

71

The result of value is a snapshot, we need 1 more step to get the value. This should be like:

const eventref = this.db.ref('cats/whiskers');
const snapshot = await eventref.once('value');
const value = snapshot.val();

answered Dec 08 '17 at 11:29

Hank Phung

Totally saved me! – zeckdude Sep 02 '18 at 03:46
What if you need to use the value afterwards, will this tell your code to wait for `value` to be assigned something before proceeding? – Badrush Dec 04 '18 at 19:13
@Badrush for your reference https://firebase.google.com/docs/reference/js/firebase.database.Query#once – Hank Phung Dec 05 '18 at 06:19
@Badrush `value` event can be used to retrieve the current value of a specific node in your database using `once` or can be used to subscribe to changes on a specific node in your database using `on`. – Hank Phung Dec 05 '18 at 06:25
What I mean is. Let's say I need to retrieve the snapshot and store it inside of `var A`... then I have a code that takes `var A` and displays it. Is there a way to tell my code to wait until the snapshot is finished retrieving so that `var A` wont be `undefined`? I do know you can use `.then` but sometimes that makes the code very messy. Especially if `var A` isn't used until much later in the code. – Badrush Dec 05 '18 at 17:04
2

@Badrush you can just use await: `var A = await eventref.once('value');` because what you're talking about is exactly what `await` do. – Hank Phung Dec 06 '18 at 09:22
Thank you. Will it still continue processing code that doesn't require `var A` in this example? – Badrush Dec 06 '18 at 16:37
@ShajeelAfzal just wrap the block in try-catch – Hank Phung Oct 21 '19 at 08:20

1 Answers1