为什么我的 Firebase 脚本挂在终端上？

Question

I'm writing a script to auto update firebase as I need to run this once a week

I have a index.js file and im running node index.js to run it, all good

I've commented out lots of the code but for now I'm just running

const calculateWinnings = () => {
    console.log('called me!')
    return firebaseApp
        .database()
        .ref(`data`)
        .once('value')
        .then((snapshot) => {
            console.log('snaph::', snapshot.val())
            return
        })
}


calculateWinnings()

this is doing what I expect in terms of it's running the function and returning me data from firebase

however, the script hangs and never exits in the terminal with the cursor marker showing

when I comment out the firebase stuff, it does what I expect

so I'm wondering how do I exit the script successfully or why is it hanging? I'm return the promise so it should be resolving?

Answer 1

The Firebase SDK performs all network and disk activity in a background thread. When the calculateWinnings() returns, the then((snapshot) => { hasn't actually been called yet. For this reason it is good that the program doesn't exit yet, or you'd never see the console.log('snaph::', snapshot.val()) .

You can see this in action, by forcing the program to exit:

const calculateWinnings = () => {
    console.log('called me!')
    return firebaseApp
        .database()
        .ref(`data`)
        .once('value')
        .then((snapshot) => {
            console.log('snaph::', snapshot.val())
            return
        })
}

calculateWinnings()
process.exit();

If you run it like this, you'll see that the snapshot from the database is never logged.

This is also the answer to how to handle it correctly: since you already return the promise from once() from your calculateWinnings method, you can wait for that to complete and then tell the process to exit:

calculateWinnings().then(() => {
  process.exit();
});

Or if you're on a more modern Node.js version:

const calculateWinnings = async () => {
  ... the function body is the same as you currently have
}

await calculateWinnings()
process.exit();