You can do it but will have to be done programmatically and not implicitly through the action attribute. For that, you can use then/catch from Promises or try/catch with async/await like the example below:

<form>
      <input type="text" name="fname" />
      <br />

      <input type="text" name="lname" />
      <br />

      <button type="submit">Send</button>
    </form>

    <script>
      const form = document.querySelector('form');

      form.addEventListener('submit', async (event) => {
        event.preventDefault();

        const elements = event.target;
        const firstName = elements['fname'].value;
        const lastName = elements['lname'].value;
        const dataToSend = { firstName, lastName };

        try {
          const response = await fetch('/example', {
            method: 'POST',
            body: JSON.stringify(dataToSend),
            headers: { 'Content-Type': 'application/json' },
          });
          const data = await response.json();

          console.log('Data returned from the server', data);
        } catch (error) {
          console.log('Uups! Something went wrong', error);
        }
      });
    </script>

Assume that I have a form as such that is submitted to a POST route /example:

<form action="/example" method="POST">
  <input type="text" name="fname"><br>
  <input type="text" name="lname"><br>
  <button type="submit">Send</button>
</form>

app.js:

app.post('/example', (req, res) => {
  ... 
});

server.js

const app = require('./app');

app.listen(3000, () => {
    console.log('server is running on port 3000');
});

Does Javascript provide a way to prevent the form submission if the server disconnects in order to prevent the website from crashing?

I have searched online but couldn’t find any such solutions to this problem.