How to Check if Variable is Undefined or Null in Javascript

In this video tutorial, you will learn how to check if variable is undefined or null in javascript.

HTML:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <link rel="stylesheet" href="style.css">
    <title>Document</title>
</head>
<body>
    
    <div>
        <button>Check</button>
        <h1>Result</h1>
    </div>

    <script src="script.js"></script>
</body>
</html>

 

CSS:

body {
    text-align: center;
}

div {
    display: inline-block;
}

button {
    display: inline-block;
    padding: 10px 20px;
}

 

Javascript:

let user1 = null;
let user2;

let btnCheck = document.querySelector('button');
let result = document.querySelector('h1');

btnCheck.addEventListener('click', () =>{
    result.innerText = user2 == undefined ? 'True' : 'False';
});

 

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