Storing objects in IndexedDB in HTML5
IndexedDB has a concept of “Object Stores.” You can think of this as a typical database table.Object stores have data (obviously) but also a keypath and an optional set of indexes. Keypaths are basically unique identifiers for your data and come in a few different formats. Indexes will be covered later when we start talking about retrieving data.
Now for something crucial. Remember the upgradeneeded event mentioned before? You can only create object stores during an upgradeneeded event. Now – by default – this will run automatically the first time a user hits your site. You can use this to create your object stores. The crucial thing to remember is that if you ever need to modify your object stores, you’re going to need to upgrade the version (back in that open event) and write code to handle your changes. Lets take a look at a simple example of this in action.
var idbSupported = false;
var db;
document.addEventListener(“DOMContentLoaded”, function(){
if(“indexedDB” in window) {
idbSupported = true;
}
if(idbSupported) {
var openRequest = indexedDB.open(“test_v2”,1);
openRequest.onupgradeneeded = function(e) {
console.log(“running onupgradeneeded”);
var thisDB = e.target.result;
if(!thisDB.objectStoreNames.contains(“firstOS”)) {
thisDB.createObjectStore(“firstOS”);
}
}
openRequest.onsuccess = function(e) {
console.log(“Success!”);
db = e.target.result;
}
openRequest.onerror = function(e) {
console.log(“Error”);
console.dir(e);
}
}
},false);
This example (test4.html) builds upon the previous entries so I’ll just focus on what’s new. Within the upgradeneeded event, I’ve made use of the database variable passed to it (thisDB). One of the properties of this variable is a list of existing object stores called objectStoreNames. For folks curious, this is not a simple array but a “DOMStringList.” Don’t ask me – but there ya go. We can use the contains method to see if our object store exists, and if not, create it. This is one of the few synchronous functions in IndexedDB so we don’t have to listen for the result.
To summarize then – this is what would happen when a user visits your site. The first time they are here, the upgradeneeded event fires. The code checks to see if an object store, “firstOS” exists. It will not. Therefore – it is created. Then the success handler runs. The second time they visit the site, the version number will be the same so the upgradeneeded event is not fired.
Now imagine you wanted to add a second object store. All you need to do is increment the version number and basically duplicate the contains/createObjectStore code block you see above. The cool thing is that your upgradeneeded code will support both people who are brand new to the site as well as those who already had the first object store. Here is an example of this (test5.html):