Uncaught typeerror: cannot set property 'onclick' of null

Dung Do Tien Jun 14 2022 258

Hi everyone, I'm a beginner in Html & Javascript. I created a login form simple and add an external javascript file into the HTML file, I watch a video guide to learn HTML & javascript, and I can import a javascript file into the HTML page inside <head> tag by using <script> tag. like this:

index.html

 <!DOCTYPE html>
<html>
    <head>
        <title>Calculator</title>
        <script src="site.js"></script>
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
    </head>
    <body>
       <input type="text" id="txtUsername" name="username" placeholder="Username" /> <br /><br />
       <input type="password" id="txtPassword" name="password" placeholder="******" /><br/><br />
       <button type="button" id="btnLogin">Login</button>
    </body>
</html>

site.js

 document.getElementById("btnLogin").onclick = function () {
    let username = document.getElementById("txtUsername").value;
    let pass = document.getElementById("txtPassword").value;
    if(!username)
    {
     alert("Enter username");
     return;
    }

    if(!pass)
    {
     alert("Enter password");
     return;
    }
    alert("Your username: " + username);
}

But when run the code above on the browser and I got an exception Uncaught TypeError: Cannot set properties of null (setting 'onclick').

 Uncaught TypeError: Cannot set properties of null (setting 'onclick')
    at site.js:1:45
(anonymous) @ site.js:1

Anyone can help me?

Have 1 answer(s) found.
  • D

    Disha Jun 14 2022

    By default, the html will load synchronous, the browser will read from begin to end of the Html page (tag by tag).

    Because your javascript file (site.js) imported inside <head> tag. So this file will read first and DOM is not ready and also does not contain btnLogin button.

    Solution: add the site.js file to the bottom of <body> tag. Like this:

     <!DOCTYPE html>
    <html>
        <head>
            <title>Login</title>
            <meta name="viewport" content="width=device-width, initial-scale=1.0">
        </head>
        <body>
           <input type="text" id="txtUsername" name="username" placeholder="Username" /> <br /><br />
           <input type="password" id="txtPassword" name="password" placeholder="******" /><br/><br />
           <button type="button" id="btnLogin">Login</button>
           <script src="site.js"></script> <!-- Should be added javascript file here-->
        </body>
    </html>

    You should learn more about DOM in javascript. It's very important to learn good javascript.

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