Expected catch() or return (promise/catch-or-return)

Question

I am new to JavaScript.This is my first function in javascript to deploy a function on the firebase.

Got this error:

 - [eslint] Unexpected function expression. (prefer-arrow-callback)
 - [eslint] Expected catch() or return (promise/catch-or-return)

What is wrong with this function?

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);

exports.grantSignupReward = functions.database.ref('/users/{uid}/last_signin_at')
  .onCreate(event => {
  var uid = event.params.uid;

  admin.database().ref('users/${uid}/referred_by')
    .once('value').then(function(data) {
    var referred_by_somebody = data.val();

    if (referred_by_somebody) {
      var moneyRef = admin.database()
      .ref('/users/${uid}/inventory/pieces_of_eight');

      moneyRef.transaction(function(current_value) {
        return (current_value || 0) + 50;
      });
    }
  });
});

Answer 1

The first error suggests you to use an arrow function as a callback. So you need to replace the regular functions, (function() { ... }), with arrow functions, (() => { ... }).

The second error suggests that you either need to catch the promise rejection, or return the promise itself. I am not too sure about your code but I believe that this method:

admin.database().ref('users/${uid}/referred_by').once('value')

returns a promise. So it needs to be returned like this:

return admin.database().ref('users/${uid}/referred_by').once('value')

or handle the error like this:

admin.database().ref('users/${uid}/referred_by').once('value')
  // ... your code here
  .catch(error => { ... });

As @Bergi pointed out in the comments that returning the promise is not preferable here, you may just add a catch block to your promise.

Answer 2

In order to solve promise/catch-or-return, just add a .catch(() => null) at the end of the line. And of course, don't forget to actually implement the error handling logic at some point in the not-so-distant future.