Weather forecasts with DOM API, fetch and JS classes

In this exercise we display weather forecasts. We use JavaScript to make the presentation interactive: download the data from the internet and display it dynamically.
DOM API
The DOM API allows JavaScript to completely control (edit, add, remove) the HTML objects constituting the web page. Please look at the example in documentation, we will be using similar operations.
We start, as usual, with the page in exercise/index.html. Please edit the files exercise/exercise_weather.js and exercise/style_weather.css to complete the exercise.
    • On Click - The page contains a button with the id btn-part1. Find it and register a JS function to be executed when the button is clicked. For now, the function should write “The button was clicked” to the console, so we can check if it executes properly upon click.
Note - Ensure the DOM has been loaded: we should make sure that the HTML has been loaded (and therefore, the button and other elements already exist) before we execute our script. Use the provided function whenDocumentLoaded to achieve that (about DOMContentLoaded event).
    • Show the temperatures - Given an array of forecast temperatures (such as the array TEST_TEMPERATURES), we want to display them in order. Create a function showTemperatures(container_element, temperature_array) such that:
        ◦ Clears the container_element’s content, for example by setting the innerHTML to “”.
        ◦ For each temperature value in temperature_array, it creates a <p> element with the temperature value inside (for example <p>13</p>). Please practoce a functional iteration method instead of an explicit for/while loop.
        ◦ Places the <p>s in container_element.
Use that function to display the temperatures from TEST_TEMPERATURES in the div with id weather-part1.
    • Colors with CSS classes - Change the background color of the <p> depending on the temperature value. Create the classes warm and cold in exercise/style_weather.css with appropriate background colors. In your showTemperatures function, apply the appropriate CSS class to the created elements:
        ◦ warm for temperature 23 C or higher,
        ◦ cold for temperature 17 C and lower.

JS Classes
Classes allow us to organize our program around objects (“object-oriented programming”) and makes code easier to reuse - do the same actions with many objects. An object contains data (called properties) and functions (called methods).
First, we convert the code from the previous part into a class.
    • Create a Forecast class, which has the following properties:
        ◦ container - the element in which the temperatures will be written, the initial value should be given as an argument to the constructor,
        ◦ temperatures - current value of the temperature forecast, should be initialized to [1,2,3,4,5,6,7].
Thus we create the object as new Forecast(container_div). Instantiate the class, giving it the div with id weather-part2 as the container.
    • Add a toString method which writes its attributes to a string.
    • Add a print method which prints that string to the console.
    • Adapt showTemperatures to create a show method which takes no arguments and instead displays the object’s temperatures in its container.
    • Add a reload method which will be loading and displaying our data:
        ◦ sets the temperatures property to TEST_TEMPERATURES (we will change it to real data later),
        ◦ calls show.
    • Upon the click of the button, call the reload method. Check if the temperatures are properly displayed.
Inheritance (extends) allows us to create a sub-type with different behaviour (for example class Dog extends Animal). In our case, we will extend the Forecast class to change its data source: the child class ForecastOnline will download its data from the internet to provide a real forecast.
    • Create the class ForecastOnline which overrides the reload method and sets the temperature to [2, 3, 4, 5, 6, 7, 8] (we will get the real data in the next section). Instantiate ForecastOnline with div with id weather-part3 and test if it shows the temperatures.

fetching JSON data from the internet
The data displayed in a visualization is most often not part of the code. Instead it is stored in data files and downloaded separately from our server or an external service.
This data can be downloaded using the Fetch API. Fetching is an asynchronous operation (we have to wait some time for the response), therefore it uses JS Promises. Please read the linked documentation to see the examples.
We can get a forecast in the JSON format using the API of www.weatherbit.io (either you use the following key or you make a free registration to the API and you use your own key), for example the query for Lausanne is:
const QUERY_LAUSANNE = 'http://api.weatherbit.io/v2.0/forecast/daily?city=Lausanne&days=7&key=ed330abe3f5a4104afd9a6ef10b707ca';
    • Use fetch to download the JSON from this URL, convert the JSON to a JS object and store the result to a global variable. Experiment with that value in the developer console to find out how to get the temperatures.
    • Create a function weatherbitToTemperatures which will extract the temperatures from the data. Since the API provides low and high temperature bounds, please calculate the mean of those values and return it as the result.
    • In ForecastOnline’s reload method, download the forecast from the specified URL and store it in the temperatures property, then call show. Please remember that show has to be executed after the data is downloaded (when the promise fires).
Optional: Interactive choice of the city
If you feel confident with DOM, fetch and classes, let’s combine them all into a more advanced system.
    • Create a class ForecastOnlineCity which allows fetching the forecast for any city:
        ◦ Add some mechanism of inputting the city name, for example a method setCity.
        ◦ Override reload to fetch the URL for the chosen city.
        ◦ Override show to also display the name of the city. You can use super.show() to call the method in the parent class. insertBefore can be useful to insert the city name before the temperature.
    • Upon the click of the button with id btn-city, read the .value of the input with id query-city, then use your ForecastOnlineCity object to perform the query.