Debugging and Troubleshooting JavaScript Code

In addition to debugging, there are several other techniques that you can use to troubleshoot your JavaScript code.

Testing Your Code

Testing your code is a crucial part of the development process. It helps you ensure that your code is correct and works as expected under different conditions. There are several types of tests that you can use to validate your code, including unit tests, integration tests, and acceptance tests.

Unit tests are small, isolated tests that validate a specific piece of code, such as a function or a module. They are typically fast to run and allow you to test individual components in isolation.

Integration tests are tests that validate the interaction between multiple components. They are typically slower to run than unit tests and may require more setup and configuration.

Acceptance tests are tests that validate the overall functionality of your code from the perspective of the user. They may involve simulating user input and verifying that the expected output is produced.

To create tests for your JavaScript code, you can use a testing framework such as Jest or Mocha. These frameworks provide tools and conventions for writing and running tests and generating test reports.

Debugging Asynchronous Code

Asynchronous code is code that executes concurrently with the rest of your program. JavaScript uses an event-driven, single-threaded model to handle asynchronous code, which means that asynchronous code is executed in a separate execution context and can be triggered by events such as user input, timers, or network requests.

Debugging asynchronous code can be challenging because the order in which the code is executed may not be obvious. To debug asynchronous code, you can use the browser’s developer console to view the call stack and set breakpoints. You can also use the console.log function to output intermediate values and trace the execution of your code.

Handling Errors

Errors are inevitable in software development, and it’s important to handle them gracefully in your code. JavaScript provides several mechanisms for handling errors, including the try and catch statements and the throw statement.

The try and catch statements allow you to catch and handle exceptions that are thrown by your code. The try block contains the code that may throw an exception, and the catch block contains the code that will be executed if an exception is thrown.

try {
  // code that may throw an exception
} catch (error) {
  // code to handle the exception
}

The `throw` statement allows you to throw an exception manually. This can be useful for signaling an error condition or for controlling the flow of your code.

if (x > 100) {
  throw new Error("x is too big!");
}

When handling errors, it’s important to choose the appropriate level of granularity. You should try to catch specific errors that you can handle and let other errors propagate up the call stack. You should also consider whether the error is recoverable or whether the program should terminate.

Debugging Third-Party Code

Sometimes you may encounter errors in third-party code, such as a library or a framework. Debugging third-party code can be challenging because you may not have access to the source code or documentation.

To debug third-party code, you can try the following techniques:

• Use the browser’s developer console to inspect the state of your variables and the call stack.
• Use the console.log function to output intermediate values and trace the execution of the code.
• Use breakpoints to pause the execution of the code and inspect the state.
• Search online for documentation or solutions to common issues.
• Reach out to the maintainers of the code and ask for help.

Conclusion

Debugging and troubleshooting are important skills in software development. By using the tools and techniques discussed in this chapter, you can identify and fix issues in your JavaScript code and improve the quality and reliability of your code.

Exercises

To review these concepts, we will go through a series of exercises designed to test your understanding and apply what you have learned.

function sum(a, b) {
  console.log(`a: ${a}, b: ${b}`);
  const result = a + b;
  console.log(`result: ${result}`);
  return result;
}

console.log(sum(1, 2)); // output: 3
console.log(sum(5, 7)); // output: 12

function divide(a, b) {
  try {
    if (b === 0) {
      throw new Error("Cannot divide by 0");
    }
    return a / b;
  } catch (error) {
    console.error(error);
  }
}

console.log(divide(4, 2)); // output: 2
console.log(divide(4, 0)); // output: "Cannot divide by 0"

function parseInt(str) {
  console.log(`str: ${str}`);
  const result = Number.parseInt(str, 10);
  console.log(`result: ${result}`);
  return result;
}

console.log(parseInt("123")); // output: 123
console.log(parseInt("hello")); // output: NaN

function sortArray(arr) {
  console.log(`arr: ${arr}`);
  const result = arr.sort((a, b) => a - b);
  console.log(`result: ${result}`);
  return result;
}

console.log(sortArray([5, 3, 1, 4, 2])); // output: [1, 2, 3, 4, 5]
console.log(sortArray([100, 50, 0, 25, 75])); // output: [0, 25, 50, 75, 100]

function fetchData(url) {
  console.log(`url: ${url}`);
  return fetch(url)
    .then(response => response.json())