In this lesson, you'll call an API by submitting your browser extension's form and displaying the results in your browser extension. In addition, you'll learn about how you can store data in your browser's local storage for future reference and use.
By this time you have built the HTML for the form and results `<div>` for your browser extension. From now on, you'll need to work in the `/src/index.js` file and build your extension bit by bit. Refer to the [previous lesson](../1-about-browsers/README.md) on getting your project set up and on the build process.
Next, add event listeners to the form and the clear button that resets the form, so that if a user submits the form or clicks that reset button, something will happen, and add the call to initialize the app at the bottom of the file:
✅ Notice the shorthand used to listen for a submit or click event, and how the event it is passed to the handleSubmit or reset functions. Can you write the equivalent of this shorthand in a longer format? Which do you prefer?
Before moving on, it's useful to learn about a very important concept available in browsers: [LocalStorage](https://developer.mozilla.org/docs/Web/API/Window/localStorage). LocalStorage is a useful way to store strings in the browser as a `key-value` pair. This type of web storage can be manipulated by JavaScript to manage data in the browser. LocalStorage does not expire, while SessionStorage, another kind of web storage, is cleared when the browser is closed. The various types of storage have pros and cons to their usage.
> Note - your browser extension has its own local storage; the main browser window is a different instance and behaves separately.
You set your APIKey to have a string value, for example, and you can see that it is set on Edge by "inspecting" a web page (you can right-click a browser to inspect) and going to the Applications tab to see the storage.
✅ Think about situations where you would NOT want to store some data in LocalStorage. In general, placing API Keys in LocalStorage is a bad idea! Can you see why? In our case, since our app is purely for learning and will not be deployed to an app store, we will use this method.
Notice that you use the Web API to manipulate LocalStorage, either by using `getItem()`, `setItem()`, or `removeItem()`. It's widely supported across browsers.
Create a function called `handleSubmit` that accepts an event argument `(e)`. Stop the event from propagating (in this case, we want to stop the browser from refreshing) and call a new function, `setUpUser`, passing in the arguments `apiKey.value` and `region.value`. In this way, you use the two values that are brought in via the initial form when the appropriate fields are populated.
```JavaScript
function handleSubmit(e) {
e.preventDefault();
setUpUser(apiKey.value, region.value);
}
```
✅ Refresh your memory - the HTML you set up in the last lesson has two input fields whose `values` are captured via the `const` you set up at the top of the file, and they are both `required` so the browser stops users from inputting null values.
Moving on to the `setUpUser` function, here is where you set local storage values for apiKey and regionName. Add a new function:
```JavaScript
function setUpUser(apiKey, regionName) {
localStorage.setItem('apiKey', apiKey);
localStorage.setItem('regionName', regionName);
loading.style.display = 'block';
errors.textContent = '';
clearBtn.style.display = 'block';
//make initial call
displayCarbonUsage(apiKey, regionName);
}
```
This function sets a loading message to show while the API is called. At this point, you have arrived at creating the most important function of this browser extension!
Before going further, we should discuss APIs. APIs, or [Application Programming Interfaces](https://www.webopedia.com/TERM/A/API.html), is a critical element of a web developer's toolbox. They provide standard ways for programs to interact and interface with each other. For example, if you are building a web site that needs to query a database, someone might have created an API for you to use. While there are many types of APIs, one of the most popular is a [REST API](https://www.smashingmagazine.com/2018/01/understanding-using-rest-api/).
✅ The term 'REST' stands for 'Representational State Transfer' and features using variously-configured URLs to fetch data. Do a little research on the various types of APIs available to developers. What format appeals to you?
There are important things to note about this function. First, notice the [`async` keyword](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Statements/async_function). Writing your functions so that they run asynchronously means that they wait for an action, such as data being returned, to be completed before continuing.
[](https://youtube.com/watch?v=YwmlRkrxvkk "Async and Await for managing promises")
- following best practices, you use an `async` keyword to make this function behave asynchronously. The function contains a `try/catch` block as it will return a promise when the API returns data. Because you don't have control over the speed that the API will respond (it may not respond at all!), you need to handle this uncertainty by calling it asynchronously.
- you're querying the co2signal API to get your region's data, using your API Key. To use that key, you have to use a type of authentication in your header parameters.
- once the API responds, you assign various elements of its response data to the parts of your screen you set up to show this data.
- if there's an error, or if there is no result, you show an error message.
✅ Using asynchronous programming patterns is another very useful tool in your toolbox. Read [about the various ways](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Statements/async_function) you can configure this type of code.
Congratulations! If you build your extension (`npm run build`) and refresh it in your extensions pane, you have a working extension! The only thing that isn't working is the icon, and you'll fix that in the next lesson.
We've discussed several types of API so far in these lessons. Choose a web API and research in depth what it offers. For example, take a look at APIs available within browsers such as the [HTML Drag and Drop API](https://developer.mozilla.org/docs/Web/API/HTML_Drag_and_Drop_API). What makes a great API in your opinion?
You learned about LocalStorage and APIs in this lesson, both very useful for the professional web developer. Can you think about how these two things work together? Think about how you would architect a web site that would store items to be used by an API.
