JavaScript Tutorials

How to handle errors in JavaScript

August 25, 2024

Error handling is a crucial aspect of JavaScript development. When errors occur, it’s important to manage them effectively to ensure a smooth user experience.

In JavaScript, we can use try-catch blocks, throw custom error messages, and employ asynchronous error handling to catch and manage exceptions.

Understanding error types in JavaScript and using tools for debugging JavaScript can further enhance the robustness of your code.

Mastering these error handling best practices not only prevents uncaught errors but also improves the overall quality of your applications. Let’s dive into the methods and strategies to handle errors in JavaScript.

try…catch Construct

maxresdefault How to handle errors in JavaScript

Syntax and Basic Usage

Structure of try and catch

The try...catch construct is essential for error handling in JavaScript. It allows you to write a block of code to test for errors while specifying what to do if an error occurs.

try {
  // Code to try
} catch (error) {
  // Code to handle the error
}

The try block contains the code that may throw an exception. The catch block handles the exception if one is thrown.

Execution flow

When an error occurs in the try block, JavaScript looks for a catch block to handle it. If no error occurs, the catch block is ignored. This ensures that normal code execution isn’t halted unless an error is thrown.

Examples

Errorless example

In the absence of errors, the catch block won’t be executed.

try {
  console.log("This runs perfectly.");
} catch (error) {
  console.log("This will not run.");
}

Example with an error

When an error is thrown, JavaScript executes the catch block.

try {
  undefinedFunction();
} catch (error) {
  console.log("An error occurred: " + error.message);
}

Runtime vs. Syntactic Errors

Definition and differences

Runtime errors: Occur during code execution, often due to invalid operations or undefined variables.
Syntactic errors: Found at compile-time due to incorrect code syntax. JavaScript won’t execute code with these errors.

Handling runtime errors

Runtime errors can be caught using the try...catch construct, allowing you to implement custom error handling and ensure your application remains functional.

try {
  const result = 10 / 0;
  console.log(result);
} catch (error) {
  console.log("Runtime error: " + error.message);
}

Limitation with syntactic errors

Syntax errors prevent code from running and cannot be caught by try...catch.

try {
  eval("let message 'Hello, world!';"); // Missing equals sign
} catch (error) {
  console.log("This will also not run.");
}

Synchronous Error Handling

Scheduled code exceptions occur during deliberate interruptions in code execution like setTimeout.

try…catch within asynchronous code

While the try...catch construct works well for synchronous code, handling errors in asynchronous code requires a different approach.

try {
  setTimeout(() => {
    throw new Error("Async error");
  }, 1000);
} catch (error) {
  console.log("This will not catch async errors.");
}

In the example above, the catch block won’t handle the asynchronous error. Instead, you need to handle errors within the callback function.

setTimeout(() => {
  try {
    throw new Error("Async error");
  } catch (error) {
    console.log("Caught async error: " + error.message);
  }
}, 1000);

Error Object

Properties of the Error Object

Understanding the Error Object in JavaScript is key to effective error handling. When exceptions are thrown, they’re represented by an error object containing useful information for debugging.

name

The name property indicates the type of error. Examples include TypeError, ReferenceError, and SyntaxError.

try {
  null.f();
} catch (error) {
  console.log(error.name); // Outputs: TypeError
}

message

The message property contains a human-readable description of the error.

try {
  undefinedFunction();
} catch (error) {
  console.log(error.message); // Outputs the error description
}

stack

The stack property provides a stack trace, showing where the error occurred. This is especially helpful for debugging.

try {
  (() => { throw new Error("Something went wrong!"); })();
} catch (error) {
  console.log(error.stack); // Outputs the stack trace
}

Custom Error Handling

Creating custom error types helps to manage specific error scenarios in your JavaScript codebase, enhancing your overall error handling strategy.

Creating custom error objects

You can create custom error objects using the built-in Error constructor or by extending the Error class to define your own error types.

class MyCustomError extends Error {
  constructor(message) {
    super(message);
    this.name = "MyCustomError";
  }
}

Utilizing built-in error constructors

JavaScript provides several built-in error constructors, like TypeError, RangeError, and ReferenceError, for more specific error handling.

try {
  throw new TypeError("This is a type error!");
} catch (error) {
  console.log(error.name); // TypeError
  console.log(error.message); // This is a type error!
}

Example of Error Object Usage

Using the Error Object effectively in practical examples can make your JavaScript error handling robust.

JSON.parse example

Errors often occur during parsing operations. For example, JSON.parse can throw a SyntaxError if the input string is not valid JSON.

try {
  JSON.parse("invalid JSON string");
} catch (error) {
  console.log(error.name); // SyntaxError
  console.log(error.message); // Outputs the error message
}

Accessing error properties

Accessing the name, message, and stack properties of an error object can provide comprehensive information about the error, making debugging simpler.

try {
  throw new Error("A critical error occurred!");
} catch (error) {
  console.log("Error Name: " + error.name);
  console.log("Error Message: " + error.message);
  console.log("Stack Trace: " + error.stack);
}

Throwing Custom Errors

The throw Statement

Handling errors effectively sometimes means creating and throwing custom errors. The throw statement offers a way to do this by specifying what very specific error to generate.

Syntax of throw

The syntax is straightforward:

throw expression;

Here, the expression can be of any type, though typically, you’ll throw an instance of the Error object or its descendants.

Types of throwables: Number, String, Boolean, Object

In JavaScript, you can throw various types of values, including numbers, strings, booleans, and objects.

Number
```
throw 404;
```
String
```
throw "This is a string error";
```
Boolean
```
throw true;
```

Object

throw { name: "CustomError", message: "An error occurred" };

Practical Examples

Understanding how to throw custom errors in practical scenarios can significantly improve your error handling strategy.

Throwing custom errors

Throwing custom errors helps to pinpoint specific issues in your code.

function checkAge(age) {
  if (age < 18) {
    throw new Error("Age must be at least 18.");
  }
  return true;
}

try {
  checkAge(15);
} catch (error) {
  console.log(error.message); // Outputs: Age must be at least 18.
}

Integrating throw with try…catch

Integrating the throw statement within a try...catch block allows for more controlled error management.

try {
  throw new Error("Something went wrong!");
} catch (error) {
  console.log(error.name); // Error
  console.log(error.message); // Something went wrong!
}

Creating Custom Error Types

Creating custom error types can provide more granularity and control over managing specific error scenarios.

Using built-in Error constructors

JavaScript’s built-in error constructors like TypeError, RangeError, and ReferenceError can be quite useful.

try {
  throw new TypeError("This is a type error!");
} catch (error) {
  console.log(error.name); // TypeError
  console.log(error.message); // This is a type error!
}

Custom error messages and names

By extending the built-in Error class, you can create custom error types with specific messages and names.

class ValidationError extends Error {
  constructor(message) {
    super(message);
    this.name = "ValidationError";
  }
}

try {
  throw new ValidationError("Invalid input!");
} catch (error) {
  console.log(error.name); // ValidationError
  console.log(error.message); // Invalid input!
}

The finally Block

Purpose and Functionality

In JavaScript, the finally block is a crucial part of error handling. It ensures that certain code executes regardless of whether an error occurred or not.

Ensuring code execution

The whole point of finally is to guarantee that some code runs no matter what happens in the try or catch blocks. This is ideal for tasks that need to be performed whether an error is thrown or the code executes successfully.

Cleanup operations

One primary use case for finally is cleanup operations, such as closing database connections, stopping timers, or freeing up resources.

Syntax and Structure

Let’s look at how to structure the finally block within a try...catch construct.

Placement in try…catch construct

The finally block is placed after the catch block. Here’s a basic structure:

try {
  // Code that might throw an error
} catch (error) {
  // Code to handle the error
} finally {
  // Code that runs no matter what
}

Example of finally usage

Consider an example where I’m fetching user data. Regardless of success or failure, I’ll want to log that the attempt was made.

try {
  // Simulate fetch operation
  let userData = fetchData();
  console.log(userData);
} catch (error) {
  console.error("Error fetching data: " + error.message);
} finally {
  console.log("Fetch operation attempted.");
}

Interaction with return Statements

Understanding how finally interacts with return statements is crucial for proper error handling.

Overriding return values

If a finally block contains a return statement, it will override any return values from the try or catch blocks.

function testFinally() {
  try {
    return "From try";
  } catch (error) {
    return "From catch";
  } finally {
    return "From finally";
  }
}

console.log(testFinally()); // Outputs: "From finally"

Ensuring cleanup despite returns

Even if the try or catch block returns a value, the finally block will still execute. This behavior ensures that cleanup code runs, providing more robust error handling.

function processData() {
  try {
    return "Processing data";
  } catch (error) {
    console.error("Error occurred");
  } finally {
    console.log("Cleanup done");
  }
}

console.log(processData()); // Outputs: "Processing data"
                            // Followed by: "Cleanup done"

Advanced Error Handling Techniques

Rethrowing Errors

Effective error handling often requires rethrowing errors after catching and processing them.

Concept and purpose

Rethrowing errors is useful when you catch an error but still want it to be handled by higher-level error processing. This way, you can log or process the error locally and then let the global error handler or another higher-level mechanism deal with it.

Example of rethrowing errors

Here’s how you might catch an error, log it, and then rethrow it:

try {
  // Simulating an error
  throw new Error("Initial error");
} catch (error) {
  console.error("Caught an error: " + error.message);
  throw error; // Rethrow the error
}

Differentiating error types in catch

When catching multiple types of errors, you can differentiate them using conditional checks, allowing specific processing for each error type before potentially rethrowing them.

try {
  // Simulate different types of errors
  let result = JSON.parse("invalid JSON");
} catch (error) {
  if (error instanceof SyntaxError) {
    console.error("JSON Syntax Error: " + error.message);
  } else if (error instanceof TypeError) {
    console.error("Type Error: " + error.message);
    throw error; // Rethrow TypeError
  } else {
    console.error("General Error: " + error.message);
    throw error; // Rethrow other errors
  }
}

Nesting try…catch Statements

Structuring nested error handling

Nested try...catch statements allow you to handle errors at different levels in a fine-tuned manner. The outer try...catch can handle errors that are either rethrown or not caught by the inner ones.

Examples of nested try…catch

Here’s an example showing nested error handling:

try {
  try {
    // Simulating an inner error
    throw new Error("Inner error");
  } catch (innerError) {
    console.error("Caught inner error: " + innerError.message);
    throw innerError; // Rethrow to outer try...catch
  }
} catch (outerError) {
  console.error("Caught outer error: " + outerError.message);
}

Global Error Handling

For those “last resort” situations where you need to catch errors that went unhandled, global error handling comes into play.

Using window.onerror in browsers

In the browser environment, the window.onerror handler allows you to catch uncaught exceptions globally.

window.onerror = function (message, source, lineno, colno, error) {
  console.error("Global error caught: " + message + " at " + source + ":" + lineno + ":" + colno);
  return true; // Prevent the default browser error handling
};

Handling uncaught exceptions

Handling uncaught exceptions ensures your application can gracefully deal with unexpected errors without crashing.

process.on('uncaughtException', function (error) {
  console.error("Uncaught exception: " + error.message);
  // Perform necessary cleanup
});

Practical Applications and Examples

Validating User Input

When it comes to validating user input, error handling becomes crucial. Here’s an example that demonstrates number range validation.

Example with number range validation

Imagine you need to validate that a user’s age falls within a specific range. If it doesn’t, an error should be thrown.

function validateAge(age) {
  if (age < 18 || age > 100) {
    throw new RangeError("Age must be between 18 and 100.");
  }
  return true;
}

try {
  validateAge(15);
} catch (error) {
  console.error("Validation Error: " + error.message);
}

Combining HTML validation with JavaScript

HTML’s form validation can be enhanced with JavaScript to provide a more robust solution.

<form id="ageForm">
  <input type="number" id="ageInput" min="18" max="100" required>
  <button type="submit">Submit</button>
</form>

<script>
  document.getElementById("ageForm").addEventListener("submit", function(event) {
    event.preventDefault();
    try {
      let age = parseInt(document.getElementById("ageInput").value, 10);
      validateAge(age);
      console.log("Age is valid: " + age);
    } catch (error) {
      console.error("Age is invalid: " + error.message);
    }
  });
</script>

Debugging with Error Objects

Proper debugging practices involve leveraging error objects to their fullest extent.

Logging errors to the console

When an error is caught, logging it to the console with detailed information is often the first step in debugging.

try {
  someNonExistentFunction();
} catch (error) {
  console.error("Error caught:", error);
}

Utilizing stack traces

The stack property of the error object provides valuable trace information.

try {
  (() => { throw new Error("A critical error occurred!"); })();
} catch (error) {
  console.log("Stack Trace: " + error.stack);
}

Error Handling in Asynchronous Code

Handling errors in asynchronous code requires a bit more care. Let’s dive into some practical techniques.

try…catch within async functions

Using try...catch within async functions helps manage errors effectively.

async function fetchData() {
  try {
    let response = await fetch("https://some-api.com/data");
    let data = await response.json();
    console.log(data);
  } catch (error) {
    console.error("Fetch Error: " + error.message);
  }
}

fetchData();

Handling errors in setTimeout and Promises

Errors within setTimeout and Promises require their own handling mechanisms.

setTimeout(() => {
  try {
    throw new Error("Something went wrong!");
  } catch (error) {
    console.error("Timeout Error: " + error.message);
  }
}, 1000);

function fetchWithPromise() {
  return new Promise((resolve, reject) => {
    setTimeout(() => reject(new Error("Promise rejected!")), 1000);
  });
}

fetchWithPromise()
  .then(data => console.log(data))
  .catch(error => console.error("Promise Error: " + error.message));

Best Practices for Error Handling

Always Use try…catch for Critical Code

When you’re looking at how to handle errors in JavaScript, it’s imperative to use try...catch blocks for critical code segments. This approach ensures that your key functionalities remain protected, even when unexpected errors occur.

Protecting key functionality

Let’s say you have functions that are crucial to the app’s operation. Wrapping these in a try...catch block can safeguard against crashes or unexpected behavior.

function performCriticalOperation() {
  try {
    // Key functionality
    executeCriticalTask();
  } catch (error) {
    console.error("Critical Operation Failed: " + error.message);
  }
}

Graceful degradation

It’s essential to degrade gracefully in the face of errors. This means offering an alternative solution or notifying the user without causing a complete failure.

function loadUserProfile() {
  try {
    fetchUserProfile();
  } catch (error) {
    displayFallbackProfile();
    console.warn("Loading default profile due to error: " + error.message);
  }
}

Provide Meaningful Error Messages

Error messages should offer clarity both to the end user and the developer. This not only helps improve user experience but also aids in debugging.

User-friendly error handling

Generic error messages can be confusing. Instead, provide messages that can direct users on what went wrong and possibly how they can rectify it.

try {
  processUserInput(input);
} catch (error) {
  alert("There was an issue processing your input. Please try again.");
}

Detailed logging for developers

Error logs need to be detailed to assist in troubleshooting. Including the name, message, and stack trace can be invaluable.

try {
  readFile("config.json");
} catch (error) {
  console.error("Error Details: ", error.name, error.message, error.stack);
}

Avoid Overusing try…catch

While try...catch blocks are essential, overusing them can introduce new problems, both in terms of performance and code readability.

Performance considerations

Excessive use of try...catch can lead to performance hits, particularly in performance-critical sections of code.

try {
  for (let i = 0; i < data.length; i++) {
    // Highly iterative code
  }
} catch (error) {
  console.error(error);
}
// Instead, consider handling errors outside the loop.

Code readability and maintenance

Placing try...catch blocks everywhere can make your code hard to read and maintain. Use them judiciously in parts where errors are most likely to occur and have the most significant impact.

function mainOperation() {
  try {
    initializeProcess();
    performMainTask();
  } catch (error) {
    console.error("Main operation error: " + error.message);
  }
}

FAQ On How To Handle Errors In JavaScript

How do you handle errors in JavaScript?

To handle errors, use try-catch blocks to catch exceptions. This helps manage errors gracefully without breaking the application. Inside the try block, place code that might throw an exception.

If an error occurs, the catch block will handle it with an error object for debugging.

What is a try-catch block?

A try-catch block is a construct to handle exceptions. The try block lets you write risky code, while the catch block handles any errors.

This way, uncaught errors don’t crash your application, allowing you to display custom error messages or perform other error handling tasks.

How can I throw a custom error in JavaScript?

Use the throw statement to generate a custom error message. For instance, throw new Error('Custom error message') creates an Error object with the message you specify. This helps in indicating specific issues occurring within your application, making debugging easier.

What types of errors can occur in JavaScript?

Three main types of errors can occur: syntax errors, runtime errors, and logical errors. Syntax errors occur at compile-time, runtime errors happen while executing the code, and logical errors are flaws in the logic that produce incorrect results but don’t throw exceptions.

How can you handle asynchronous errors in JavaScript?

Asynchronous error handling can be managed using promises and the .catch method or by using async/await with try-catch blocks. For promises, attach a .catch to handle rejections. Using async/await, wrap the asynchronous code in try-catch to catch any errors.

What is an uncaught error?

An uncaught error is an exception that wasn’t handled by any try-catch block or error handling mechanism. This type of error can crash your application. Implementing global error handlers and catching errors before they propagate helps to manage these situations.

How do you debug JavaScript?

Debugging involves using browser tools like Chrome DevTools to inspect code. Set breakpoints, step through code, and view logged JavaScript errors. Additionally, tools such as linters and integrated development environments (IDEs) provide real-time error detection and suggestions.

What is an error object in JavaScript?

An Error object provides information about an error that occurred. It has properties like message and name that describe the error. You can create custom Error objects using new Error('message'), which helps in generating specific error messages for better debugging.

How do you use the finally block?

The finally block runs after the try-catch execution, regardless of whether an error occurred or not. It is useful for cleaning up resources, like closing connections or files, ensuring that essential code runs no matter what happens in the try or catch blocks.

Why is proper error handling important?

Proper JavaScript error handling ensures a smooth user experience, prevents application crashes, and aids in debugging.

It allows you to show meaningful error messages, log issues for later review, and maintain robust software by addressing potential vulnerabilities and bugs effectively.

Conclusion

How to handle errors in JavaScript is a fundamental skill for ensuring robust and user-friendly applications. Utilizing try-catch blocks, generating custom error messages, and mastering asynchronous error handling can significantly improve the reliability of your code.

Effective error management involves understanding error types in JavaScript, leveraging debugging tools, and implementing error handling best practices. Whether it’s handling runtime errors, managing promise rejections, or using a finally block for cleanup, these techniques pave the way for smoother user experiences and more maintainable code.

Incorporate these strategies to minimize uncaught errors and enhance your JavaScript development.

Author
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Bogdan Sandu

Bogdan is a seasoned web designer and tech strategist, with a keen eye on emerging industry trends.

With over a decade in the tech field, Bogdan blends technical expertise with insights on business innovation in technology.

A regular contributor to TMS Outsource's blog, where you'll find sharp analyses on software development, tech business strategies, and global tech dynamics.