return from forEach in node.js to loop through firebase results

Question

How can I get a return after I loop through all my firebase results to properly finish my cloud function?

var count = 0;
return ref.child('/records').once('value').then(snap=>{
    snap.forEach(snapChild=>{
        var ucName = ${snapChild.val().name.toUpperCase();
        var update = {'name':ucName);
        ref.child(`/records/${snapChild.key}`).update(update).then(()=>{
            count++;
            res.write(`<p>${ucName} updated</p>`);
        });
    })
}).then(()=>{
    return res.end(`<p>end of list (${count} records)</p>`);
})

It actually does what it is supposed to do but the counter stays at 0 and I get an error 'write after end' - I guess due to the missing return from forEach.

I have a question unrelated to your problem @alexmac answered that. Why use ES6 (fat arrow) but do not use const and let ? — Orelsanpls
– Orelsanpls, Commented Sep 22, 2017 at 11:40
You are right - I should have for ucName and update - no doubt. Just not so clear about count - as it is not 'constant'; I need to read up on it after I am 'finished' with firebase, cloud functions, node, nodemailer, handlebars, auth, Polymer, PWA.... ;) — MGR Programming
– MGR Programming, Commented Sep 22, 2017 at 21:22
count is a variable that is changing of value due to count++, it is a let. ucName and update in other hand do not get modified, they are const. Good luck with theses techs :) — Orelsanpls
– Orelsanpls, Commented Sep 22, 2017 at 22:59

alexmac · Accepted Answer · 2017-09-22 11:26:14Z

1

It's because the last then callback is called, when snap children is not processed. You need to use Array#map to produce an array of promises, and Promise.all to wait until all promises are not resolved:

var count = 0;
return ref
  .child('/records').once('value')
  .then(snap => {
    let ops = snap.map(snapChild => {
        var ucName = ${snapChild.val().name.toUpperCase();
        var update = {'name':ucName);
        return ref.child(`/records/${snapChild.key}`).update(update).then(() => {
            count++;
            res.write(`<p>${ucName} updated</p>`);
        });
    });
    return Promise.all(ops);
  })
  .then(()=>{
    return res.end(`<p>end of list (${count} records)</p>`);
  });

answered Sep 22, 2017 at 11:26

alexmac

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1 Comment

MGR Programming Over a year ago

Thanks - I was on the path of Promise.all but did not figure it out. Thanks for the solution.

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