// set
chrome.storage.promise.local.set({'foo': 'bar'}).then(function() {
  // resolved
  console.log('set');
}, function(error) {
  // rejected
  console.log(error);
});
 
// get
chrome.storage.promise.local.get('foo').then(function(items) {
  // resolved
  console.log(items); // => {'foo': 'bar'}
}, function(error) {
  // rejected
  console.log(error);
});