As part of the big Counter Example project, in this article we'll see how to create a counter with multiple buttons with plain JavaScript utilizing the Local Storage introduced in HTML5.

In HTML5 localStorage is a term used for a flat key-value database inside the browser (or more specifically on the hard-disk of the computer, table, or smartphone where the browser runs) that can be accessed using JavaScript.

examples/js/multi_counter.html

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1, user-scalable=yes">
  <title>Multi counter</title>
</head>
<body>
<div id="counter"></div>
<button id="reset_all">Reset All</button>

<div id="counters">
</div>

<input id="name"><button id="add_counter">Add counter</button>

<div id="status"></div>

<script>
let counters = {};


function load_counters() {
    let counters_str = localStorage.getItem('multi_counter');
    if (counters_str === null) {
        console.log('new multi counter system');
    } else {
        counters = JSON.parse(counters_str);
    }
    show_counters();
}

function increase_counter(event) {
    let name = event.target.getAttribute("data-name");
    counters[name]++;
    localStorage.setItem('multi_counter', JSON.stringify(counters));
    show_counters();
}

function show_counters() {
    let counter_list =  Object.keys(counters);
    console.log(counter_list);
    let html = "";
    for (let ix = 0; ix < counter_list.length; ix++) {
        let name = counter_list[ix];
        html += `<button class="counter" data-name="${name}">${name} : ${counters[name]}</button> `;
    }
    document.getElementById('counters').innerHTML = html;
    let elements = document.getElementsByClassName('counter');
    console.log(elements);
    for (let ix = 0; ix < elements.length; ix++) {
        elements[ix].addEventListener('click', increase_counter);
    }
}


function reset_all() {
    document.getElementById('status').innerHTML = '';
    counters = {};
    localStorage.removeItem('multi_counter');
    show_counters();
}

function add_counter() {
    let name = document.getElementById('name').value;
    // remove spaces
    name = name.replace(/^\s+/g, "");
    name = name.replace(/\s+$/g, "");
    if (name == "") {
        document.getElementById('status').innerHTML = 'Missing name for new counter';
        return;
    }
    // console.log(counters[name]);
    if (name in counters) {
        document.getElementById('status').innerHTML = `counter '${name}' already exists`;
        return;
    }
    counters[name] = 1;
    localStorage.setItem('multi_counter', JSON.stringify(counters));
    document.getElementById('status').innerHTML = 'added';
    document.getElementById('name').value = "";
    show_counters();
}

document.getElementById('reset_all').addEventListener('click', reset_all);
document.getElementById('add_counter').addEventListener('click', add_counter);
load_counters();
show_counters();
</script>
</body>
</html>
Try!

Click on the Try! link to see how it works in a separate window.