How do I debug a Nightmare script?

Debugging a Nightmare script, which is a high-level browser automation library for Node.js, involves several strategies to understand what's happening in your code and the browser it's controlling. Here are some methods to debug a Nightmare script:

1. Enable Nightmare's Verbose Logging

Nightmare has built-in verbose logging that you can enable to see what's happening under the hood. To enable verbose logging, you can set the DEBUG environment variable when running your script. In the terminal, prepend your script execution command with DEBUG=nightmare*:

DEBUG=nightmare* node yourscript.js

This will print out a lot of information about the actions Nightmare is taking and any Electron messages.

2. Visual Debugging

Nightmare offers options to open the Electron window so you can see what's happening. You can also take screenshots or save the HTML of the page at various stages to get a snapshot of the state at that point in time.

To show the Electron window, set the show option to true:

const Nightmare = require('nightmare');
const nightmare = Nightmare({ show: true });

nightmare
  .goto('https://example.com')
  // ... your actions
  .catch(error => {
    console.error('An error occurred:', error);
  });

To take a screenshot:

nightmare
  .goto('https://example.com')
  // ... your actions
  .screenshot('/path/to/your/screenshot.png')
  .catch(error => {
    console.error('An error occurred:', error);
  });

To save the HTML:

nightmare
  .goto('https://example.com')
  // ... your actions
  .evaluate(() => document.documentElement.innerHTML)
  .then(html => {
    console.log(html);
  })
  .catch(error => {
    console.error('An error occurred:', error);
  });

3. Using .then() and .catch()

Nightmare uses promises, so you can use .then() and .catch() to handle successful execution and errors respectively.

nightmare
  .goto('https://example.com')
  // ... your actions
  .then(() => {
    console.log('Done!');
  })
  .catch(error => {
    console.error('An error occurred:', error);
  });

4. Console Output within the Page Context

You can also use console.log within the browser context by using the .evaluate() method. However, you'll need to capture these logs manually since they won't show up in the Node.js console by default.

nightmare
  .on('console', (log, msg) => {
    console.log(`Console log from page: ${msg}`);
  })
  .goto('https://example.com')
  .evaluate(() => {
    console.log('This message is from inside the page context!');
  })
  // ... your actions
  .then(() => {
    // ...
  });

5. Breakpoints and Node Inspector

You can use Node's built-in debugging capabilities to set breakpoints and step through your code. Run your script with --inspect or --inspect-brk to enable debugging and then use Chrome DevTools to connect to the Node process.

node --inspect-brk yourscript.js

Open Chrome and navigate to chrome://inspect/ to attach the debugger to your Node process.

6. Checking Network Requests

Sometimes, it's helpful to see the network requests being made by the page.

nightmare
  .on('did-get-response-details', function() {
    console.log(arguments);
  })
  .goto('https://example.com')
  // ... your actions

7. Additional Debugging Modules

There are modules like nightmare-har-plugin which can provide HAR (HTTP Archive) recordings of the session, which is useful for analyzing network activity during your automation.

Conclusion

By using these debugging techniques, you should be able to gain insight into what's happening in your Nightmare scripts and troubleshoot any issues that arise. Debugging is an iterative process, so combine these methods as needed to solve the specific problems you encounter.