Cannot set variable of undefined

https://stackoverflow.com/questions/38807643/error-uncaught-in-promise-typeerror-cannot-set-property-isadmin-of-null

when variable not set in function "then" block, this doesnt work, need to a copy of "this" reference with in the calling function.

down voteaccepted

Either you can use arrow function firebase.database().ref('/userProfile/' + user.uid).once('value') .then((snapshot) => { or use var self = this; firebase.database().ref('/userProfile/' + user.uid).once('value') .then(function(snapshot) { self.isAdmin = true; otherwise this doesn't point to the current function when it's called. See also https://developer.mozilla.org/de/docs/Web/JavaScript/Reference/Functions/Arrow_functions