React native firebase how to assign value when getting data from db

Question

Hi I am trying to assign var foo to a value within my firebase databse, however this just gives me undefined when using console log on foo.

var foo;
  firebase.database().ref(`/topics/lec1`).once('value').then(function(snapshot) {
     foo = snapshot.val();

    console.log(foo); 

  });

   console.log(foo); //undefined

Just one question, is first console.log is also returning undefined? One inside the then? if not, then it's okay. You are logging foo before it is set by firebase since it is asynchronous — Tanmay_vijay
– Tanmay_vijay, Commented Apr 1, 2019 at 20:02
first you need to assign it, second its gonna be undefined outside then(), since firebase is asynchronous. — Peter Haddad
– Peter Haddad, Commented Apr 1, 2019 at 20:03

JavanPoirier · Accepted Answer · 2019-04-01 19:44:28Z

1

Nowhere are you assigning the foo variable. Try below:

  var foo;
      firebase.database().ref(`/topics/lec1`).once('value').then(function(snapshot) {
         foo = snapshot.val();
     this.state.lec  = foo;
        console.log(foo); 

      });

       console.log(foo); //undefined

answered Apr 1, 2019 at 19:44

JavanPoirier

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2 Comments

jimmy Over a year ago

still undefined

JavanPoirier Over a year ago

Instead of function() use a anonymous arrow function: () => {

Tanmay_vijay · Accepted Answer · 2019-04-01 20:09:13Z

0

Do whatever you want to do inside the callback

var foo;
firebase
  .database()
  .ref(`/topics/lec1`)
  .once("value")
  .then(function(snapshot) {
    foo = snapshot.val();
    displayValue();
  });

function displayValue() {
  //use value from firebase via foo.
}

answered Apr 1, 2019 at 20:09

Tanmay_vijay

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2 Comments

jimmy Over a year ago

Thank you soo much!

Frank van Puffelen Over a year ago

In React you'd typically call setState(...) from within displayValue, which will then trigger an update of the UI.

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