Read a singe key value from IndexedDB

readKey.js

// Assuming https://github.com/jakearchibald/idb-keyval has been used for key-value entries,
// the following is about the minimum code needed to read the value for a single key (w/o the lib.)
// Usage: `readKey('name').then(function(name) { console.log(name); });`

function readKey(key) {
    var indexedDB = window.indexedDB || window.mozIndexedDB || window.webkitIndexedDB || window.msIndexedDB || window.shimIndexedDB;
    if (!indexedDB) return Promise.reject(new Error('IndexedDB not available'));
    return new Promise(function(resolve, reject) {
        var open = indexedDB.open('keyval-store', 1);
    
        open.onerror = function () {
            reject(open.error);
        };
    
        open.onupgradeneeded = function () {
            open.result.createObjectStore('keyval');
        };
    
        open.onsuccess = function() {
            var db = open.result;
            var tx = db.transaction('keyval', 'readonly');
            var store = tx.objectStore('keyval');
            var getKey = store.get(key);

            getKey.onsuccess = function() {
                resolve(getKey.result);
            };

            tx.onerror = function () {
                reject(tx.error);
            };
            
            tx.oncomplete = function () {
                db.close();
            };
        };
    });
}