Chapter 05 Chapter: Debugging and Troubleshooting Userscripts





Console Logging



Console logging is an essential tool for debugging JavaScript and userscripts because it allows developers to track the execution of their code and identify any issues or errors that may arise during runtime. Here are a few reasons why console logging is important:

1. Error detection: Console logging helps in detecting errors or exceptions in your code. By strategically placing log statements at various points in your script, you can track the flow of execution and identify where something goes wrong. When an error occurs, the console will display the relevant log messages, making it easier to pinpoint the cause of the issue.

2. Code flow analysis: Logging key variables, function calls, or control flow decisions helps you understand how your code is behaving at different stages of execution. By inspecting the logged values in the console, you can verify if the code is following the expected path and identify any unexpected behavior.

3. Data inspection: During development, it's crucial to inspect the values of variables, objects, or arrays to ensure they hold the expected data. By logging these values, you can quickly check if the data is correct and troubleshoot any inconsistencies.

4. Performance monitoring: Console logging can also be used to measure the performance of your code. By logging timestamps or other relevant information at specific points, you can analyze the execution time of different sections and identify potential bottlenecks that might impact the overall performance of your script.

5. Tracking control flow: When working with complex scripts, it can be challenging to keep track of the control flow and understand how different functions or modules interact with each other. Console logging allows you to follow the execution path and understand how your code is working step by step.

Console logging provides a powerful means to gather information about the execution of your JavaScript or userscripts. It enables you to detect errors, analyze code flow, inspect data, monitor performance, and understand the control flow of your code. By utilizing console logging effectively, you can streamline the debugging process and develop more robust and reliable scripts.


Simple example of console logging:

function calculateSum(a, b) {
  console.log('Calculating sum...');
  console.log('a:', a);
  console.log('b:', b);
  
  const sum = a + b;
  
  console.log('Sum:', sum);
  
  return sum;
}

const result = calculateSum(5, 3);
console.log('Result:', result);


In this example, we have a function `calculateSum` that takes two arguments `a` and `b`. Inside the function, we log messages and values using `console.log()` to track the flow of execution and inspect the values of `a`, `b`, and the calculated sum. Finally, we log the result of the function call outside the function.

When you run this code in a JavaScript environment that supports console logging (e.g., a browser's developer console or Node.js), you will see the following output:

Calculating sum...
a: 5
b: 3
Sum: 8
Result: 8


This example demonstrates how console logging can be used to track the execution of the code, inspect variable values, and verify the correctness of calculations.

> Note you can wrap large blocks of code in `try-catch` containers to catch exceptions




Breakpoints and Step-Through Debugging



You can set breakpoints and step-through code in userscripts. However, the availability and functionality of debugging tools may vary depending on the environment or browser extension you are using to execute the userscript.

Here's a general overview of how you can set breakpoints and perform step-through debugging in userscripts using popular browser extensions like Tampermonkey or Greasemonkey:

1. Install a browser extension: First, you'll need to install a browser extension like Tampermonkey (for Chrome) or Greasemonkey (for Firefox). These extensions allow you to run userscripts and provide additional debugging features.

2. Open the userscript in the extension's dashboard: Once you have the extension installed, open its dashboard and find the userscript you want to debug. The dashboard usually provides a list of installed userscripts and an editor for each script.

3. Set breakpoints: In the userscript editor, you can set breakpoints by placing the `debugger;` statement at the desired line of code. When the script reaches that line during execution, it will pause, and you can inspect variables, step through the code, and analyze its behavior.

4. Enable debugging mode: In some extensions, you may need to enable a debugging mode or specific settings for the userscript to enable breakpoints and step-through debugging.

5. Run the userscript: Once you have set breakpoints and enabled debugging, run the userscript on the target web page. The script execution will pause when it reaches a breakpoint.

6. Inspect variables and step through code: At the breakpoint, you can use the debugging tools provided by the extension to inspect variables, view call stacks, and step through the code. Typically, you'll have options to step into functions, step over lines of code, or step out of a function to continue execution.

7. Analyze and troubleshoot: By stepping through the code, you can observe how variables change, identify potential issues, and narrow down the cause of bugs or unexpected behavior in your userscript.

>> It's important to note that the exact steps and features for debugging userscripts may vary based on the specific browser extension you're using. Therefore, it's recommended to refer to the documentation or resources provided by the extension's developers for detailed instructions on how to utilize their debugging capabilities.






Error Handling and Exception Catching




Error handling and exception catching are important techniques in userscripts to gracefully handle and recover from unexpected errors or exceptions that may occur during script execution. They help prevent script failures and allow for more robust and reliable userscripts. Here's a brief overview of error handling and exception catching in userscripts:

1. Error types: JavaScript has built-in error types, such as `Error`, `TypeError`, `ReferenceError`, `SyntaxError`, and more. These errors represent different types of exceptional conditions that can occur during script execution.

2. Try-catch statement: The `try-catch` statement is used to handle exceptions and errors in JavaScript. The `try` block contains the code that may potentially throw an exception, while the `catch` block defines how to handle the exception if it occurs.

3. Catching specific exceptions: In the `catch` block, you can specify the type of exception you want to catch by including the corresponding error type as a parameter. For example:

try {
  // Code that may throw an exception
} catch (error) {
  // Handle the exception
}


4. Handling exceptions: In the `catch` block, you can define the actions to be taken when an exception occurs. This can include logging an error message, displaying a user-friendly error notification, or performing any necessary cleanup tasks.

5. Finally block: Optionally, you can include a `finally` block after the `try-catch` block. The code inside the `finally` block will be executed regardless of whether an exception was thrown or caught. It is commonly used to perform cleanup operations.

6. Throwing exceptions: You can manually throw an exception using the `throw` statement. This allows you to create custom error conditions and handle them appropriately.

Here's an example of error handling and exception catching in a userscript:

try {
  // Code that may throw an exception
  const result = someFunction();
} catch (error) {
  // Handle the exception
  console.error('An error occurred:', error);
  // Perform additional error handling or recovery tasks
} finally {
  // Cleanup operations (optional)
  console.log('Script execution completed.');
}


In this example, the `someFunction()` call inside the `try` block may throw an exception. If an exception occurs, it will be caught in the `catch` block, and the error message will be logged to the console. You can also perform additional error handling or recovery tasks as needed. The `finally` block is used to log a completion message, indicating that the script execution has finished.

Implementing error handling and exception catching in userscripts, you can improve the resilience of your scripts and provide better user experiences by gracefully handling errors and recovering from exceptional conditions.





Common Pitfalls and How to Avoid Them



If your script is running on someone elses website there maybe unforseen effects. The website may not function correctly, your code may interfere or stop native scripts on the site functioning. You also have to be aware that websites can be active and dynamic (maybe constantly loading and unloading content in the background while you're interacting with the website).









Userscripts - Copyright Benjamin Kenwright