開啟資料庫

開啟資料庫是一種非同步操作。我們需要傳送一個請求來開啟我們的資料庫然後監聽事件，以便我們知道什麼時候準備就緒。

我們將開啟一個 DemoDB 資料庫。如果它還不存在，它將在我們傳送請求時建立。

下面的 2 說我們要求我們的資料庫的第 2 版。你可以隨時只使用一個版本，但我們可以使用版本號升級舊資料，如你所見。

var db = null, // We'll use this once we have our database
    request = window.indexedDB.open("DemoDB", 2);

// Listen for success. This will be called after onupgradeneeded runs, if it does at all
request.onsuccess = function() {
    db = request.result; // We have a database!

    doThingsWithDB(db);
};

// If our database didn't exist before, or it was an older version than what we requested,
// the `onupgradeneeded` event will be fired.
// 
// We can use this to setup a new database and upgrade an old one with new data stores
request.onupgradeneeded = function(event) {
    db = request.result;

    // If the oldVersion is less than 1, then the database didn't exist. Let's set it up
    if (event.oldVersion < 1) {
        // We'll create a new "things" store with `autoIncrement`ing keys
        var store = db.createObjectStore("things", { autoIncrement: true });
    }

    // In version 2 of our database, we added a new index by the name of each thing
    if (event.oldVersion < 2) {
        // Let's load the things store and create an index
        var store = request.transaction.objectStore("things");

        store.createIndex("by_name", "name");
    }
};

// Handle any errors
request.onerror = function() {
    console.error("Something went wrong when we tried to request the database!");
};