// by passing an object you can define default values e.g.: []
chrome.storage.local.get({userKeyIds: []}, function (result) {
    // the input argument is ALWAYS an object containing the queried keys
    // so we select the key we need
    var userKeyIds = result.userKeyIds;
    userKeyIds.push({keyPairId: keyPairId, HasBeenUploadedYet: false});
    // set the new array value to the same key
    chrome.storage.local.set({userKeyIds: userKeyIds}, function () {
        // you can use strings instead of objects
        // if you don't  want to define default values
        chrome.storage.local.get('userKeyIds', function (result) {
            console.log(result.userKeyIds)
        });
    });
});