Creating a TODO list can be considered just a little bit further from "Hello World", the "standard" example people use when learning a new language. In this example we'll build a very basic TODO application using some HTML5 features and JavaScript.

The very first thing we need is to create an HTML page. We could get by with just the following 3 lines, as those are the important lines, but I thought adding a few extra lines of HTML would make it more "correct".

The 3 important lines are the following: 

<input id="task"><button id="add">Add</button>
<div id="todos"></div>
<script src="todo.js"></script>

At first we have an input element where we'll be able to enter text and it has a button that we'll be able to click.

In the second row we have an empty div element. We are going to display the current list in that element.

Finally we load an external JavaScript file called todo.js

The reason we load the JavaScript file at the end is that we wanted to make sure the other elements are already in the browser by the time the JavaScript file is loaded and by the time it starts to run.

The full version can be seen here, and you can even try it by clicking on the link below.

examples/js/todo.html

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes">
</head>
<body>
<input id="task"><button id="add">Add</button>
<hr>
<div id="todos"></div>

<script src="todo.js"></script>
</body>
</html>
Try!

In the JavaScript code we have 4 function, and after the declaration of those 3 function we have the following code: 

document.getElementById('add').addEventListener('click', add);
show();

The first line locates the HTML element that has the id "add" using the getElementById method. On the returned object we call the addEventListener method and assign the add function to the click event. This code will make sure then whenever the user clicks on the "Add" button, the add function will be called.

Finally we run the show function.

The 4 function we have in our code are the following:

show will display the current list of TODO items.

add will take the text from the input box and save it in our "database".

remove will remove the selected item from the list of TODO items in our "database".

get_todos is the function that will retrieve the list of TODO items from our "database".

The "database"

I put the word in quotes as it is not really a database in the same way most people consider databases, but then any place we can persistently store data could be called a "database".

Specifically we are going to use the localStorage facility. It is a key-value pair database in the browser. We can store any string in it and that data will be available to us when we return to the same page later. We just have to remember, the information stored in localStorage is not going to be sent to the server and it won't be available on the same page if we visit it from another computer, or even from the same computer, but a different browser.

For our TODO list we are going to use a single key in the localStorage and the value will be the stringified version of the list of TODO items we have.

Fetching from the database: get_todos

As this function used by all the other functions, let's see the get_todos function first. It does not get any parameter. It just fetches the content of the todo key of the localStorage using the getItem method. If this is the first time ever the function is called, the specific localStorage entry will be empty and the localStorage.getItem call will return null. In that case we return the newly created empty Array.

If the returned value is not null then it must be the stringified data we stored earlier. We use JSON.parse to convert the JSON string back to JavaScript data and return that. 

function get_todos() {
    var todos = new Array;
    var todos_str = localStorage.getItem('todo');
    if (todos_str != null) {
        todos = JSON.parse(todos_str); 
    }
    return todos;
}

Adding a new TODO entry

The second function we might want to take a look at is the one called add which is called when the user has clicked on the All button. At first, using getElementById it locates the HTML element with the id task which is the input box and then it retrieves the value the user has typed in.

Then, calling get_todos we retrieve the already existing list of TODO items from the "database". As explained above, at the first time this function will return an empty Array.

We append the new task to the Array using the push method and then save the new list of TODO items in the "database". For this we first stringify the Array using the JSON.stringify method and then we store the returned string using the localStorage.setItem method.

In the next step we call the show() function that will update the list of TODOs displayed on the web page.

Finally we return false; to avoid any further actions generated by the 'click' event. 

function add() {
    var task = document.getElementById('task').value;

    var todos = get_todos();
    todos.push(task);
    localStorage.setItem('todo', JSON.stringify(todos));

    show();

    return false;
}

show the TODO list

The show function will display the current TODO list stored in the "database". First thing it calls get_todos to get the (possibly empty) Array of TODO items.

Then we manually create an HTML snippet in the, otherwise arbitrarily named html variable. This is a ul element (and unordered list), with a li (list item) for each TODO entry. In addition to the content of the todos array we also add a button to each list item. Each button belongs to a class called 'remove' and each button has an id containing the index of the todo item in the list retrieved from the "database". We'll use these buttons to allow the user to remove an item from the list.

The call document.getElementById('todos').innerHTML = html; insert the newly generated HTML snippet in the original document loaded from the server. It actually replaces the content of the element with the id "todos". This means in subsequent calls it will just show the new list regardless of what was there earlier.

In the next 4 lines we use the getElementsByClassName method to fetch all the buttons that are in the 'remove' class. These are the buttons we have just added to each todo item. To each button we assign a event listener that will be called if the user clicks on either of those buttons. The call to addEventListener connects the 'click' event to the remove function. 

function show() {
    var todos = get_todos();

    var html = '<ul>';
    for(var i=0; i<todos.length; i++) {
        html += '<li>' + todos[i] + '<button class="remove" id="' + i  + '">x</button></li>';
    };
    html += '</ul>';

    document.getElementById('todos').innerHTML = html;

    var buttons = document.getElementsByClassName('remove');
    for (var i=0; i < buttons.length; i++) {
        buttons[i].addEventListener('click', remove);
    };
}

remove a TODO item

Finally we arrive to the remove function that will be called when the user clicks on any of the remove buttons. (The remove buttons have an X on them.)

this represents the current DOM-object which is the remove-button the user just clicked. We retrieve the value of its id attribute using the getAttribute method. This is the index of the specific TODO item among the TODO items in the "database".

After retrieving the current list of TODO items, we use the splice method to remove a specific element from the JavaScript array, and then we store the new list back the database.

Then, just as in the add function we call the show function to update the list in the browser as well and we return false; to stop the propagation of the 'click' event. 

function remove() {
    var id = this.getAttribute('id');
    var todos = get_todos();
    todos.splice(id, 1);
    localStorage.setItem('todo', JSON.stringify(todos));

    show();

    return false;
}

Full JavaScript code for TODO list

That's it all the code for the JavaScript based TODO list.

examples/js/todo.js

function get_todos() {
    var todos = new Array;
    var todos_str = localStorage.getItem('todo');
    if (todos_str !== null) {
        todos = JSON.parse(todos_str); 
    }
    return todos;
}

function add() {
    var task = document.getElementById('task').value;

    var todos = get_todos();
    todos.push(task);
    localStorage.setItem('todo', JSON.stringify(todos));

    show();

    return false;
}

function remove() {
    var id = this.getAttribute('id');
    var todos = get_todos();
    todos.splice(id, 1);
    localStorage.setItem('todo', JSON.stringify(todos));

    show();

    return false;
}

function show() {
    var todos = get_todos();

    var html = '<ul>';
    for(var i=0; i<todos.length; i++) {
        html += '<li>' + todos[i] + '<button class="remove" id="' + i  + '">x</button></li>';
    };
    html += '</ul>';

    document.getElementById('todos').innerHTML = html;

    var buttons = document.getElementsByClassName('remove');
    for (var i=0; i < buttons.length; i++) {
        buttons[i].addEventListener('click', remove);
    };
}

document.getElementById('add').addEventListener('click', add);
show();