unexpected identifier in javascript, but I have declared the variable

Question

I've looked at this file for an hour and have tried a ton of permutations.

$(document).ready(function() {

var unreadMessages = document.getElementsByClassName("unreadMessage");

var previousEntries;

chrome.storage.sync.get(null, {
    var previousEntries;
    previousEntries = (data["item"]);
});

console.log(previousEntries);


});

Why does previousEntries throw an "Uncaught SyntaxError: unexpected identifier"?

This seems to be the most straightforward task. Is it a } () or , issue?

Answer 1

$(document).ready(function() {

  var unreadMessages = document.getElementsByClassName("unreadMessage");

  var previousEntries;

  //                            vvvvvvvvvvvvvv
  chrome.storage.sync.get(null, function(data){
    // var previousEntries;              And if you want to access this variable outside,
    previousEntries = data["item"];   // don't redefine it here (no "var" keyword).
    logValue();                       // Now that the variable is set, you can use it
  });

  // console.log(previousEntries);    // Undefined, because it is not set yet.

  function logValue(){
    console.log(previousEntries);     // Now it should work (called from get's callback).
  }

});

The reason the variable is not set yet is that chrome.storage.sync.get is, just like Ajax or a setTimeout, asynchronous. Imagine doing this:

var myVar;

setTimeout(function(){
   myVar = 'Hello world!';
   console.log(myVar);     // logs "Hello world!" to the console
}, 100);

alert(myVar);              // alerts undefined