JavaScript-Concepts/null_vs_undefined.html at master · kipchirchiralb/JavaScript-Concepts · GitHub

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<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Null vs Undefined</title>
</head>

<body>
    <h1>Null Vs Undefined</h1>
    <p>basically both mean that there is no value. Infact equating both using loose equality sign == will return true. However, using strict equality === will return false.</p>

    <h4>So what really are their differences?</h4>

    <h3>Null</h3>
    <p> When a variable is null it means that the variable has no value and it was explicitly set to have null value by the programmer. This means a variable will never be null unles set to null in the code. </p>
    <p>For example, in a find function that queries a database for an entry. If no entry exists it makes the most sense to return null since you are stating that there is no value found.</p>

    <h3>Undefined</h3>
    <p>?Undefined means there is no value because no value has been set. If you create a variable and do not assign it a value, then it will be null.</p>

    <h4>What now? When do I use these?</h4>

    <p>Essentially you could set a variable to <em>undefined</em> when trying to convey a message that the variable no longer contains any useful information while when the value is <em>null</em> you are specifically saying the result of some action has
        no value</p>


    <p> </p>
    <script>
        let a
        console.log(a)

        let b = null
        console.log(b)

        let c = undefined
        console.log(c)
    </script>
</body>

</html>