Codepath

Debugging JavaScript with Breakpoints Tutorial
Edit PagePage History

Here’s a simple JavaScript example that you can run and debug using breakpoints. This code simulates a user interacting with a form, and we’ll set breakpoints to inspect the state of the program during execution. You can use this to step through the code.

Example Code: Debugging a Simple Form Submission

Create a new HTML file and paste the following code into it:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Debugging with Breakpoints</title>
</head>
<body>
    <h1>Simple Form</h1>
    <form id="myForm">
        <label for="name">Name:</label>
        <input type="text" id="name" name="name" required>
        <br>
        <label for="email">Email:</label>
        <input type="email" id="email" name="email" required>
        <br>
        <button type="submit">Submit</button>
    </form>

    <script>
        // Form submission handler
        document.getElementById('myForm').addEventListener('submit', function(event) {
            event.preventDefault();  // Prevent page refresh

            const name = document.getElementById('name').value;
            const email = document.getElementById('email').value;

            console.log('Form submitted with values:');
            console.log('Name:', name);
            console.log('Email:', email);

            // Simulating an asynchronous operation (e.g., saving to a database)
            setTimeout(() => {
                console.log('Data saved successfully!');
            }, 2000);
        });

        // Some simple validation
        function validateForm() {
            const name = document.getElementById('name').value;
            if (name.trim() === '') {
                alert('Name cannot be empty!');
                return false;
            }
            return true;
        }
    </script>
</body>
</html>

How to Debug

  1. Open the Page: Open the HTML file in Google Chrome.
  2. Open DevTools: Right-click anywhere on the page and select Inspect, or press Ctrl + Shift + I (Cmd + Option + I on Mac).

  3. Set Breakpoints:

    • Open the Sources tab.
    • Find the script under file:// or localhost in the left-hand panel.
    • Set breakpoints on the lines where you want to inspect the code. For example:
      • Line 19 (where the form values are logged).
      • Line 26 (inside the setTimeout() callback).

  4. Interact with the Form: Fill out the form and click Submit. The page won't refresh due to event.preventDefault().

  5. Step Through the Code:

    • The debugger will pause at the breakpoints.
    • You can inspect the values of the name and email variables.
    • Step through the code line-by-line using the debugging controls in DevTools.

Stepping through the code

Fork me on GitHub