JavaScript

JavaScript - Async - Generation 1 - Callbacks

From
https://www.joshwcomeau.com/javascript/promises/

Sample code

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<html>
<head>
    <meta charset="utf-8">
    <title>JavaScript - Async - Level 1 - Callbacks</title>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <meta name="description" content="used by atom.xls">    
</head>
<body>
    <h1>JavaScript - Async - Level 1 - Callbacks</h1>
    
    Callbacks were the first generation of multitasking within the language. The example given is setting up a timer and keeping the concurrency running.
    
    Here is a sample of a countdown.

    <script>
        // Run JavaScript after the DOM is fully loaded
        document.addEventListener('DOMContentLoaded', function () {
            console.log('DOM fully loaded and parsed.');
            // Your script logic here
            
            console.log('Start');
            console.log("3…");

            setTimeout(() => {
                console.log("2…");
                setTimeout(() => {
                        console.log("1…");
                        setTimeout(() => {
                            console.log("Happy New Year!!");
                        }, 1000);
                    console.log("1…After setTimeout");
                }, 1000);
                console.log("2…After setTimeout");
            }, 1000);
            console.log("3…After setTimeout");

            // end of event listener
        });
    </script>

    With callback you find yourself embedding code, deeper and deeper. It get's confusing as is referred to as Callback Hell.

    Since JavaScript is not a truly multitasking (or multithreaded language) the rule of setTimeout is 1000 milliseconds at a minumum. 
</body>

</html>