JavaScript Error Handling and HTML Validation: Best Practices and Examples
Types of JavaScript Errors
JavaScript is a versatile and widely used programming language for web development, but like any programming language, it can produce errors when your code contains mistakes or encounters unexpected situations. JavaScript errors can be categorized into several types, each with its own characteristics and implications.
This lesson covers essential concepts in JavaScript error handling, including try-catch blocks, common error types like ReferenceError and TypeError, and best practices for handling errors gracefully. Additionally, it explores HTML validation for web forms, ensuring data integrity and compliance with standards. Explore code examples and gain insights into effective error management and form validation techniques.
Here are some common JavaScript errors:
Syntax Errors:
These errors occur when the JavaScript engine encounters code that violates the language’s syntax rules.
Common causes include missing parentheses, brackets, or semicolons, as well as typographical errors. Syntax errors prevent the code from running at all.
Example:
if (x > 10 // Missing closing parenthesis
{
console.log(“x is greater than 10”);
}
Syntax Errors:complete code example in html
Here’s an example of an HTML document that includes a JavaScript block with a syntax error:
<!DOCTYPE html>
<html>
<head>
<title>Syntax Error Example</title>
</head>
<body>
<h1>Syntax Error Example</h1>
<script>
// This JavaScript block contains a syntax error (missing closing parenthesis)
if (x > 10 {
console.log("x is greater than 10");
}
</script>
</body>
</html>
In this example:
we have an HTML document that includes a <script> element for JavaScript code.
Inside the <script> element, there’s a if statement with a missing closing parenthesis, which is a syntax error.
When you open this HTML file in a web browser and open the browser’s developer console (usually by pressing F12 or right-clicking and selecting “Inspect”), you will see an error message indicating the syntax error:
Uncaught SyntaxError:
Unexpected token ‘{‘
The browser’s developer console will highlight the line where the syntax error occurs, making it easier for you to identify and fix the issue in your code.
In this case, you should add a closing parenthesis to the if statement to correct the syntax error:
if (x > 10) {
console.log("x is greater than 10");
}
After making this correction, the code will execute without syntax errors.
Reference Errors:
Reference errors occur when you try to use a variable or function that has not been declared or is out of scope.
This often happens when you mistype a variable name or try to access a variable that doesn’t exist in the current context.
Example:
document.write.log(nonExistentVariable); // ReferenceError: nonExistentVariable is not defined
Reference Errors:complete code example in html
Here’s an example of an HTML document that includes a JavaScript block with a reference error:
<!DOCTYPE html> <html> <head> <title>Reference Error Example</title> </head> <body> <h1>Reference Error Example</h1> <script> // This JavaScript block contains a reference error (using an undeclared variable) console.log(nonExistentVariable); </script> </body> </html>
In this example:
we have an HTML document with a <script> element containing JavaScript code.
Inside the <script> element, there’s a console.log() statement that attempts to log the value of an undeclared variable nonExistentVariable.
This will result in a reference error.
When you open this HTML file in a web browser and open the browser’s developer console, you will see an error message indicating the reference error:
Uncaught ReferenceError:
nonExistentVariable is not defined
The browser’s developer console will highlight the line where the reference error occurs, helping you identify the issue in your code. To fix this reference error, you should either declare the variable nonExistentVariable or use an existing variable within your code.
Type Errors:
Type errors occur when you perform an operation on a value of the wrong data type.
This could involve trying to call a non-function as a function, accessing a property on a non-object, or attempting arithmetic operations with incompatible data types.
Example:
var x = "5"; var y = 10; var z = x + y; // z will be "510" (string concatenation), not 15 (number addition)
Type Errors: complete code example in html
Here’s an example of an HTML document that includes a JavaScript block with a type error:
<!DOCTYPE html> <html> <head> <title>Type Error Example</title> </head> <body> <h1>Type Error Example</h1> <script> // This JavaScript block contains a type error (concatenating a string and a number) var x = "5"; var y = 10; var z = x + y; console.log(z); </script> </body> </html>
In this example:
we have an HTML document with a <script> element containing JavaScript code.
Inside the <script> element, we declare two variables, x and y. x is assigned a string value “5”, and y is assigned a numeric value 10. We then attempt to concatenate these two variables using the + operator, which will result in a type error because JavaScript tries to perform string concatenation when one of the operands is a string.
When you open this HTML file in a web browser and open the browser’s developer console, you will see the type error:
Uncaught TypeError:
Cannot convert ‘5’ to a number
The browser’s developer console will highlight the line where the type error occurs. To fix this type error, you should either ensure that both x and y are of compatible data types for the operation you want to perform or convert one of them to the appropriate data type.
For example, you can convert x to a number like this:
var x = "5"; var y = 10; var z = Number(x) + y; console.log(z);
This will result in z being 15, which is the numeric addition of x and y.
Range Errors:
Range errors occur when you try to manipulate an object with a length or size that is out of the allowed range.
For example, attempting to access an array element that doesn’t exist can result in a range error.
Example:
var arr = [1, 2, 3]; console.log(arr[5]); // RangeError: Index out of range
Range Errors:complete code example in html
Here’s an example of an HTML document that includes a JavaScript block with a range error:
<!DOCTYPE html> <html> <head> <title>Range Error Example</title> </head> <body> <h1>Range Error Example</h1> <script> // This JavaScript block contains a range error (accessing an out-of-range index in an array) var arr = [1, 2, 3]; console.log(arr[5]); </script> </body> </html>
In this example, we have an HTML document with a <script> element containing JavaScript code. Inside the <script> element, we declare an array arr containing three elements. We then attempt to access an element at index 5, which is out of the valid range for this array (indices in JavaScript arrays are zero-based).
When you open this HTML file in a web browser and open the browser’s developer console, you will see the range error:
Uncaught RangeError: Index out of range
The browser’s developer console will highlight the line where the range error occurs. To fix this range error, you should access a valid index within the bounds of the array, or you can check the length of the array before accessing an index to ensure it’s within the valid range. For example:
var arr = [1, 2, 3];
if (arr.length > 5) {
console.log(arr[5]); // This will not produce a range error
} else {
console.log("Index out of range");
}In this updated code, we check the length of the array before attempting to access an index to avoid the range error.
Custom Errors:
JavaScript also allows you to create custom errors using the Error constructor. These are useful for handling specific situations in your code and can include additional information about the error.
Example:
function divide(x, y) {
if (y === 0) {
throw new Error("Division by zero is not allowed");
}
return x / y;
}Custom Errors: complete code example in html
Here’s an example of an HTML document that includes a JavaScript block with a custom error:
In this example:
we have an HTML document with a <script> element containing JavaScript code.
Inside the <script> element, there’s a function divide(x, y) that performs division.
If the denominator (y) is 0, it throws a custom Error object with the message “Division by zero is not allowed.”
We then call the divide function with the arguments 10 and 0.
Since we’re attempting to divide by zero, an error is thrown, and we catch that error using a try…catch block. Inside the catch block, we log the error message.
When you open this HTML file in a web browser and open the browser’s developer console, you will see the custom error message:
Caught an error:
Division by zero is not allowed
Custom errors like these are helpful for providing descriptive error messages and handling specific situations in your code. You can customize the error message and structure to suit your application’s needs.
<!DOCTYPE html>
<html>
<head>
<title>Custom Error Example</title>
</head>
<body>
<h1>Custom Error Example</h1>
<script>
// This JavaScript block defines and throws a custom error
function divide(x, y) {
if (y === 0) {
throw new Error("Division by zero is not allowed");
}
return x / y;
}
try {
var result = divide(10, 0);
console.log("Result:", result);
} catch (error) {
console.error("Caught an error:", error.message);
}
</script>
</body>
</html>
Unhandled Promise Rejections:
In asynchronous JavaScript code that uses Promises, unhandled Promise rejections can occur if a Promise is rejected but no .catch() or .then(null, …) handler is attached to handle the rejection.
Example:
const promise = new Promise((resolve, reject) => {
reject("Something went wrong");
});
// This will result in an unhandled Promise rejection errorHandling errors in JavaScript involves using constructs like try…catch to gracefully handle exceptions and prevent them from crashing your application.
Proper error handling is essential for robust and reliable code.
Unhandled Promise Rejections: complete code example in html
Here’s an example of an HTML document that includes a JavaScript block with an unhandled Promise rejection:
<!DOCTYPE html>
<html>
<head>
<title>Unhandled Promise Rejection Example</title>
</head>
<body>
<h1>Unhandled Promise Rejection Example</h1>
<script>
// This JavaScript block contains an unhandled Promise rejection
const promise = new Promise((resolve, reject) => {
reject("Something went wrong");
});
// This Promise rejection is unhandled
</script>
</body>
</html>In this example:
we have an HTML document with a <script> element containing JavaScript code.
Inside the <script> element, we create a Promise that is explicitly rejected using the reject function with the message “Something went wrong.” However, we do not attach a .catch() or .then(null, …) handler to handle this rejection.
When you open this HTML file in a web browser and open the browser’s developer console, you will see an unhandled Promise rejection warning or error message.
The exact message may vary depending on the browser, but it will typically indicate that there’s an unhandled Promise rejection.
To handle this Promise rejection and prevent the unhandled rejection warning, you can add a .catch() handler to handle the rejection:
const promise = new Promise((resolve, reject) => {
reject("Something went wrong");
});
promise.catch((error) => {
console.error("Handled Promise rejection:", error);
});With this change, the Promise rejection is handled, and you will not see an unhandled Promise rejection warning in the console. Instead, the error message will be logged as a “Handled Promise rejection.”
JavaScript try and catch
The try…catch statement in JavaScript is used for handling exceptions or errors in your code. It allows you to gracefully handle errors, preventing them from crashing your program or website. Here’s how it works:
Try Block (try {…}):
You enclose the code that might throw an exception or error inside a try block. If an error occurs within this block, it will be caught by the associated catch block.
Catch Block (catch (error) {…}):
If an error occurs within the try block, control is transferred to the catch block. The catch block takes an error parameter, which represents the error object associated with the thrown exception. You can then use this object to access information about the error, such as its message or type.
Here’s a simple example:
try {
// Code that might throw an error
let x = y + 5; // This will throw a ReferenceError because 'y' is not defined
} catch (error) {
// Code to handle the error
console.error("An error occurred:", error.message);
}
In this example, a try block attempts to add 5 to a variable y, which is not defined.
This results in a ReferenceError.
The control is then transferred to the catch block, where we log an error message.
Handling Different Types of Errors
You can use multiple catch blocks to handle different types of errors, such as ReferenceError, TypeError, or custom errors:
try {
// Code that might throw different types of errors
let x = y + 5; // ReferenceError
let z = null;
z.someFunction(); // TypeError
throw new Error("Custom error occurred");
} catch (referenceError) {
console.error("ReferenceError:", referenceError.message);
} catch (typeError) {
console.error("TypeError:", typeError.message);
} catch (customError) {
console.error("Custom Error:", customError.message);
}In this example:
we have separate catch blocks for different error types. Depending on the type of error thrown in the try block, the corresponding catch block will be executed.
Finally Block (finally {…})
You can also include a finally block after the catch block.
The code in the finally block runs regardless of whether an error occurred or not.
This is often used for cleanup tasks like closing files or releasing resources:
try {
// Code that might throw an error
} catch (error) {
// Code to handle the error
} finally {
// Code that always runs, whether there was an error or not
}
The finally block is optional, and you can omit it if you don’t need to perform any cleanup operations.
The try…catch statement is a powerful tool for handling errors in JavaScript, making your code more robust and preventing it from crashing when unexpected issues arise.
Try Block (try {…}): complete code example in html
Here’s an example of an HTML document that includes a try block in a JavaScript script:
<!DOCTYPE html>
<html>
<head>
<title>Try Block Example</title>
</head>
<body>
<h1>Try Block Example</h1>
<script>
try {
// Code that might throw an error
let x = y + 5; // This will throw a ReferenceError because 'y' is not defined
} catch (error) {
// Code to handle the error
console.error("An error occurred:", error.message);
}
</script>
</body>
</html>In this example:
we have an HTML document with a <script> element containing JavaScript code.
Inside the <script> element, there’s a try block that attempts to add 5 to a variable y, which is not defined.
This will result in a ReferenceError.
If an error occurs within the try block, control is transferred to the catch block, where we log an error message.
When you open this HTML file in a web browser and open the browser’s developer console, you will see the error message:
An error occurred: y is not defined
The catch block is responsible for handling the error and displaying an appropriate message.
Catch Block (catch (error) {…}):complete code example in html
Here’s an example of an HTML document that includes a catch block in a JavaScript script to handle an error:
<!DOCTYPE html>
<html>
<head>
<title>Catch Block Example</title>
</head>
<body>
<h1>Catch Block Example</h1>
<script>
try {
// Code that might throw an error
let x = y + 5; // This will throw a ReferenceError because 'y' is not defined
} catch (error) {
// Code to handle the error
console.error("An error occurred:", error.message);
}
</script>
</body>
</html>In this example:
we have an HTML document with a <script> element containing JavaScript code.
Inside the <script> element, there’s a try block that attempts to add 5 to a variable y, which is not defined.
This will result in a ReferenceError.
If an error occurs within the try block, control is transferred to the catch block, where we log an error message.
When you open this HTML file in a web browser and open the browser’s developer console, you will see the error message:
An error occurred: y is not defined
The catch block is responsible for handling the error and displaying an appropriate message. In this case, we use console.error() to log the error message, but you can customize the error handling logic within the catch block to suit your needs.
Handling Different Types of Errors:complete code example in html
Here’s an example of an HTML document that includes a try block with multiple catch blocks to handle different types of errors:
<!DOCTYPE html>
<html>
<head>
<title>Handling Different Types of Errors</title>
</head>
<body>
<h1>Handling Different Types of Errors</h1>
<script>
try {
// Code that might throw different types of errors
let x = y + 5; // This will throw a ReferenceError because 'y' is not defined
let z = null;
z.someFunction(); // This will throw a TypeError
throw new Error("Custom error occurred"); // This will throw a custom error
} catch (referenceError) {
// Code to handle ReferenceError
console.error("ReferenceError:", referenceError.message);
} catch (typeError) {
// Code to handle TypeError
console.error("TypeError:", typeError.message);
} catch (customError) {
// Code to handle the custom error
console.error("Custom Error:", customError.message);
}
</script>
</body>
</html>
In this example:
we have an HTML document with a <script> element containing JavaScript code.
Inside the <script> element, there’s a try block that contains code that might throw different types of errors:
A ReferenceError is thrown when we attempt to add 5 to a variable y, which is not defined.
A TypeError is thrown when we try to call a method (someFunction()) on a variable z that is null.
We explicitly throw a custom error using throw new Error(“Custom error occurred”);.
We have multiple catch blocks to handle each type of error. Depending on the type of error thrown in the try block, the corresponding catch block will be executed.
When you open this HTML file in a web browser and open the browser’s developer console, you will see error messages for each type of error:
ReferenceError: y is not defined
TypeError: Cannot read property ‘someFunction’ of null
Custom Error: Custom error occurred
Each catch block is responsible for handling a specific type of error and displaying an appropriate error message.
JavaScript Throws Errors
In JavaScript, you can explicitly throw errors using the throw statement. Throwing errors allows you to handle exceptional situations in your code and provide informative error messages.
The throw statement is typically used in conjunction with try…catch blocks to catch and handle these errors. Here’s how it works:
<!DOCTYPE html>
<html>
<head>
<title>Handling Different Types of Errors</title>
</head>
<body>
<h1>Handling Different Types of Errors</h1>
<script>
try {
// Code that might throw different types of errors
let x = y + 5; // This will throw a ReferenceError because 'y' is not defined
let z = null;
z.someFunction(); // This will throw a TypeError
throw new Error("Custom error occurred"); // This will throw a custom error
} catch (referenceError) {
// Code to handle ReferenceError
console.error("ReferenceError:", referenceError.message);
} catch (typeError) {
// Code to handle TypeError
console.error("TypeError:", typeError.message);
} catch (customError) {
// Code to handle the custom error
console.error("Custom Error:", customError.message);
}
</script>
</body>
</html>In the code above:
Inside the try block, you place the code that might throw an error. In this case, we’re using an if statement to check a condition, and if that condition is not met, we throw a custom error using throw new Error().
If an error occurs within the try block, control is transferred to the catch block, where you can handle the error. The catch block takes an error parameter, which represents the error object associated with the thrown exception.
Inside the catch block, you can access information about the error, such as its message (error.message), and you can perform error handling logic as needed.
Here’s an example with a more specific error type, a RangeError, being thrown:
try {
// Code that might throw a RangeError
let number = -1;
if (number < 0) {
throw new RangeError("Number must be non-negative.");
}
// ...
} catch (error) {
// Code to handle the error
console.error("Caught a RangeError:", error.message);
}In this example: if the number is negative, a RangeError is thrown with a custom error message.
The catch block handles the RangeError specifically.
Throwing errors can be useful for signaling exceptional conditions in your code and providing context-specific error messages.
You can create and throw your own custom error types by extending the Error object or by defining custom error classes to represent different error scenarios in your application.
complete code example in html
Here’s an example of an HTML document that includes JavaScript code that explicitly throws and catches an error:
<!DOCTYPE html>
<html>
<head>
<title>Throw and Catch Error Example</title>
</head>
<body>
<h1>Throw and Catch Error Example</h1>
<script>
try {
// Code that might throw an error
let number = -1;
if (number < 0) {
throw new RangeError("Number must be non-negative.");
}
// This code will not be executed if the error is thrown
console.log("This code will not be reached.");
} catch (error) {
// Code to handle the error
console.error("Caught an error:", error.message);
}
</script>
</body>
</html>In this example:
we have an HTML document with a <script> element containing JavaScript code.
Inside the <script> element, there’s a try block that contains code that might throw a RangeError. Specifically, it checks if the number variable is less than 0, and if so, it throws a RangeError with a custom error message.
When you open this HTML file in a web browser and open the browser’s developer console, you will see the error message:
Caught an error: Number must be non-negative.
The catch block handles the RangeError by logging the error message. If the condition were not met (i.e., number is not negative), the code after the try…catch block would continue executing without errors.
nput Validation Example:complete code example
Input validation is crucial in web applications to ensure that user-provided data meets certain criteria or constraints before processing it.
Here’s a complete HTML and JavaScript example for input validation using a simple form:
<!DOCTYPE html>
<html>
<head>
<title>Input Validation Example</title>
</head>
<body>
<h1>Input Validation Example</h1>
<form id="validationForm">
<label for="age">Enter your age:</label>
<input type="text" id="age" name="age" required>
<span id="ageError" style="color: red;"></span><br>
<label for="email">Enter your email:</label>
<input type="email" id="email" name="email" required>
<span id="emailError" style="color: red;"></span><br>
<button type="submit">Submit</button>
</form>
<script>
const form = document.getElementById('validationForm');
const ageInput = document.getElementById('age');
const ageError = document.getElementById('ageError');
const emailInput = document.getElementById('email');
const emailError = document.getElementById('emailError');
form.addEventListener('submit', function (event) {
// Prevent the form from submitting by default
event.preventDefault();
// Clear previous error messages
ageError.textContent = '';
emailError.textContent = '';
// Validate age (must be a positive number)
const ageValue = parseFloat(ageInput.value);
if (isNaN(ageValue) || ageValue <= 0) {
ageError.textContent = 'Age must be a positive number.';
return;
}
// Validate email (simple check for "@" character)
const emailValue = emailInput.value;
if (!emailValue.includes('@')) {
emailError.textContent = 'Email must contain "@" character.';
return;
}
// If validation passes, you can proceed with form submission or other actions
alert('Form submitted successfully!');
});
</script>
</body>
</html>
In this example:
We create an HTML form with two input fields for age and email. We also add <span> elements to display error messages.
In the JavaScript part, we access form elements and listen for the form’s submit event.
When the form is submitted, we prevent the default form submission behavior using event.preventDefault().
We then perform input validation:
For the age input, we check if it’s a positive number.
For the email input, we check if it contains the “@” character.
If any validation errors are found, we display error messages in the respective <span> elements.
If validation passes, you can proceed with form submission or other actions. In this example, we show an alert message to indicate successful form submission.
This is a basic example of input validation, and in real-world applications, you may need more sophisticated validation logic depending on your requirements. Additionally, server-side validation is also important to ensure the integrity and security of user-submitted data.
HTML Validation
HTML validation ensures that the markup of an HTML document adheres to the rules and standards defined by the HTML specification.
It helps to identify and correct errors in the HTML code, ensuring that web pages render correctly in different web browsers and are accessible to users.
There are several tools and methods for validating HTML:
HTML Validators:
W3C Markup Validation Service:
This is a popular online tool provided by the World Wide Web Consortium (W3C), the organization that develops HTML standards. You can access it at https://validator.w3.org/. Simply enter the URL of your web page or paste your HTML code, and it will check for HTML validity and provide a detailed report of any issues.
Command-Line Validators:
There are command-line tools like html5validator and tidy that can be used for HTML validation. These tools can be integrated into build processes and scripts.
Browser Developer Tools:
Most modern web browsers come with built-in developer tools that include an HTML validator. For example, in Google Chrome, you can open the developer tools (F12 or right-click and “Inspect”) and go to the “Console” tab. Any HTML validation errors or warnings will be displayed there.
IDE and Text Editors:
Many integrated development environments (IDEs) and text editors have HTML validation built in. For example, Visual Studio Code with the “HTMLHint” extension can highlight HTML errors as you type.
Online Editors:
Online HTML editors like CodePen, JSFiddle, and JS Bin often include HTML validation as part of their feature set.
Server-Side Validation:
You can implement server-side validation using server-side programming languages like PHP, Python, or Node.js. These languages can parse and validate HTML submitted by users.
HTML validation checks for various issues, including:
Properly nested tags.
Valid attribute values.
Use of deprecated elements or attributes.
Missing or mismatched closing tags.
Unescaped special characters like <, >, &, etc.
Incorrect use of HTML5 elements, like <section> or <article>.
It’s important to note that while HTML validation helps ensure well-formed HTML, it does not necessarily guarantee that your web page will render perfectly in all browsers or that it complies with accessibility guidelines. Additional testing and validation may be required for those aspects.
HTML Validation:complete code example
HTML validation is typically performed using online tools like the W3C Markup Validation Service, but here’s an example of how you can integrate a form on a web page to collect user input and use JavaScript for simple client-side HTML validation.
This example demonstrates form input validation for a name and email address:
<!DOCTYPE html>
<html>
<head>
<title>HTML Validation Example</title>
</head>
<body>
<h1>HTML Validation Example</h1>
<form id="validationForm">
<label for="name">Name:</label>
<input type="text" id="name" name="name" required pattern="[A-Za-z ]+" title="Only letters and spaces are allowed"><br>
<label for="email">Email:</label>
<input type="email" id="email" name="email" required><br>
<button type="submit">Submit</button>
</form>
<script>
const form = document.getElementById('validationForm');
form.addEventListener('submit', function (event) {
event.preventDefault();
const nameInput = document.getElementById('name');
const emailInput = document.getElementById('email');
if (!nameInput.checkValidity()) {
alert("Please enter a valid name.");
return;
}
if (!emailInput.checkValidity()) {
alert("Please enter a valid email address.");
return;
}
// Form is valid, proceed with submission or other actions
alert("Form submitted successfully!");
});
</script>
</body>
</html>In this example:
We have an HTML form with two input fields for name and email.
We use the required attribute to make sure both fields are filled in before submission.
We also use the pattern attribute on the name input to specify that only letters and spaces are allowed.
The title attribute provides a description of the pattern requirements.
JavaScript is used to add a submit event listener to the form.
When the form is submitted, the event handler prevents the default form submission.
We then check the validity of the name and email inputs using the checkValidity() method.
If validation fails, an alert is displayed with a corresponding error message.
If both inputs pass validation, you can proceed with form submission or other actions.
In this example, we display a success message.
Please note that client-side validation can be bypassed or manipulated by users, so it should always be complemented with server-side validation for security and reliability. This example demonstrates basic client-side validation for educational purposes.
The finally Statement:complete code example in html
The finally statement is used in conjunction with the try…catch block in JavaScript to specify a block of code that will be executed regardless of whether an exception is thrown or caught. Here’s a complete HTML and JavaScript example that demonstrates the use of the finally statement:
<!DOCTYPE html>
<html>
<head>
<title>finally Statement Example</title>
</head>
<body>
<h1>finally Statement Example</h1>
<script>
try {
// Code that might throw an error
let result = 10 / 0; // This will throw a division by zero error
} catch (error) {
// Code to handle the error
console.error("Caught an error:", error.message);
} finally {
// Code that always runs, whether there was an error or not
console.log("Finally block executed.");
}
</script>
</body>
</html>In this example:
We have an HTML document with a <script> element containing JavaScript code.
Inside the <script> element, we use a try…catch block to handle an operation that might throw an error. In this case, we attempt to divide 10 by 0, which results in a division by zero error.
The catch block catches the error and logs an error message.
The finally block contains code that will always run, whether or not an error was thrown. In this case, it logs a message indicating that the “Finally block executed.”
When you open this HTML file in a web browser and open the browser’s developer console, you will see both the error message from the catch block and the message from the finally block:
Caught an error: Division by zero
Finally block executed.
The finally block is useful for performing cleanup tasks, such as closing files or releasing resources, regardless of whether an error occurred in the try block.
Error Object Properties
The Error object in JavaScript has several properties that provide information about an error. These properties can be accessed to obtain details about the error that occurred. Here are some of the common properties of the Error object:
name:
This property contains a string that specifies the name of the error. It’s usually set to the error type, such as “Error,” “ReferenceError,” “TypeError,” or a custom error type you define.
message:
This property contains a string that provides a human-readable description of the error. It typically describes what went wrong and can be customized when creating custom error objects.
stack:
This property contains a stack trace, which is a string representing the call stack at the point where the error occurred. It includes a list of function calls that led to the error and their source locations. The stack trace is particularly useful for debugging.
Here’s an example that demonstrates how to access these properties:
try {
// Code that might throw an error
throw new Error("This is a custom error message.");
} catch (error) {
console.log("Error Name:", error.name);
console.log("Error Message:", error.message);
console.log("Stack Trace:", error.stack);
}
In this example, we create a custom Error object with a specific message.
When we catch the error in the catch block, we can access its name, message, and stack properties to obtain information about the error.
Please note that some browser-specific errors or custom error types may have additional properties specific to those errors. For example, a SyntaxError object may have properties like fileName and lineNumber to indicate the source file and line number where the syntax error occurred.
Different error types in JavaScript (e.g., ReferenceError, TypeError, etc.) may also have their own specific properties in addition to the common properties mentioned above. It’s a good practice to consult the documentation for a specific error type when working with it to understand its properties and behavior.
Error Object Properties:complete code example in html
Here’s a complete HTML and JavaScript example that demonstrates how to access and display the properties of an Error object:
<!DOCTYPE html>
<html>
<head>
<title>Error Object Properties Example</title>
</head>
<body>
<h1>Error Object Properties Example</h1>
<script>
try {
// Code that might throw an error
let x = y + 5; // This will throw a ReferenceError because 'y' is not defined
} catch (error) {
// Access and display error properties
const errorName = error.name;
const errorMessage = error.message;
const errorStack = error.stack;
// Display error properties in HTML
const errorInfo = document.createElement("div");
errorInfo.innerHTML = `<p>Error Name: ${errorName}</p><p>Error Message: ${errorMessage}</p><p>Stack Trace: <pre>${errorStack}</pre></p>`;
document.body.appendChild(errorInfo);
}
</script>
</body>
</html>In this example:
We have an HTML document with a <script> element containing JavaScript code.
Inside the <script> element, we have a try…catch block that attempts to add 5 to a variable y, which is not defined. This will result in a ReferenceError.
In the catch block, we access and store the name, message, and stack properties of the caught Error object in variables.
We create a new div element called errorInfo and set its inner HTML to display the error properties, including the error name, message, and stack trace.
Finally, we append the errorInfo div to the <body> of the HTML document, which will display the error information on the web page.
When you open this HTML file in a web browser and open the browser’s developer console, you will see the error message generated by the catch block, and you will also see the error properties displayed on the web page.
The error properties provide information about the caught ReferenceError.
Eval Error:complete code example in html
The EvalError object in JavaScript represents an error that occurs when using the eval() function. Note that EvalError is rarely used in practice because modern JavaScript engines typically do not throw EvalError instances.
Instead, they throw SyntaxError when there’s a problem with eval().
Here’s a complete HTML and JavaScript example that attempts to use eval() and handles a potential EvalError:
<!DOCTYPE html>
<html>
<head>
<title>Eval Error Example</title>
</head>
<body>
<h1>Eval Error Example</h1>
<script>
try {
// Code that might throw an EvalError (rare)
const code = "console.log('Hello, world!');";
eval(code); // Attempt to evaluate the code
} catch (error) {
if (error instanceof EvalError) {
// Handle EvalError
console.error("EvalError occurred:", error.message);
} else {
// Handle other error types
console.error("An error occurred:", error.message);
}
}
</script>
</body>
</html>In this example:
We have an HTML document with a <script> element containing JavaScript code.
Inside the <script> element, we use a try…catch block to catch potential errors that may occur when using the eval() function.
We define a const variable code that contains a string of JavaScript code. We then attempt to evaluate this code using eval(code).
In the catch block, we check if the caught error is an instance of EvalError. However, it’s important to note that, in practice, modern JavaScript engines typically throw SyntaxError for issues with eval().
If an EvalError is caught, we log an error message specific to EvalError. If any other error type is caught, we log a more generic error message.
When you open this HTML file in a web browser and open the browser’s developer console, you will see the error message indicating an EvalError if the code inside eval() encounters an issue. Again, please note that EvalError is rarely used in modern JavaScript, and you’re more likely to encounter SyntaxError or other error types when working with eval().
Range Error:complete code example in html
A RangeError occurs in JavaScript when you try to manipulate a numeric value that is outside the range of valid values.
For example, trying to create an array with an invalid length or accessing an out-of-bounds index in an array can result in a RangeError.
Here’s a complete HTML and JavaScript example that demonstrates a RangeError:
<!DOCTYPE html>
<html>
<head>
<title>RangeError Example</title>
</head>
<body>
<h1>RangeError Example</h1>
<script>
try {
// Code that might throw a RangeError
let arr = new Array(-1); // Attempting to create an array with a negative length
} catch (error) {
if (error instanceof RangeError) {
// Handle RangeError
console.error("RangeError occurred:", error.message);
} else {
// Handle other error types
console.error("An error occurred:", error.message);
}
}
</script>
</body>
</html>In this example:
We have an HTML document with a <script> element containing JavaScript code.
Inside the <script> element, we use a try…catch block to catch potential RangeError exceptions.
In the try block, we attempt to create an array with a negative length by using new Array(-1). This operation is invalid and will result in a RangeError.
In the catch block, we check if the caught error is an instance of RangeError. If it is, we log an error message specific to RangeError. If any other error type is caught, we log a more generic error message.
When you open this HTML file in a web browser and open the browser’s developer console, you will see the error message indicating a RangeError due to the attempt to create an array with a negative length. This demonstrates how to handle a RangeError using a try…catch block in JavaScript.
Reference Error:
complete code example in html
A ReferenceError occurs in JavaScript when you try to reference a variable or object that has not been declared or is out of scope. Here’s a complete HTML and JavaScript example that demonstrates a ReferenceError:
<!DOCTYPE html>
<html>
<head>
<title>ReferenceError Example</title>
</head>
<body>
<h1>ReferenceError Example</h1>
<script>
try {
// Code that might throw a ReferenceError
let x = y + 5; // Attempting to use 'y' which is not defined
} catch (error) {
if (error instanceof ReferenceError) {
// Handle ReferenceError
console.error("ReferenceError occurred:", error.message);
} else {
// Handle other error types
console.error("An error occurred:", error.message);
}
}
</script>
</body>
</html>In this example:
We have an HTML document with a <script> element containing JavaScript code.
Inside the <script> element, we use a try…catch block to catch potential ReferenceError exceptions.
In the try block, we attempt to use the variable y, which has not been declared. This operation will result in a ReferenceError.
In the catch block, we check if the caught error is an instance of ReferenceError. If it is, we log an error message specific to ReferenceError. If any other error type is caught, we log a more generic error message.
When you open this HTML file in a web browser and open the browser’s developer console, you will see the error message indicating a ReferenceError due to the attempt to use the undefined variable y. This demonstrates how to handle a ReferenceError using a try…catch block in JavaScript.
URI (Uniform Resource Identifier) Error:complete code example in html
A URIError in JavaScript occurs when there’s a problem with the encoding or decoding of a Uniform Resource Identifier (URI), such as a malformed or invalid URI component.
Here’s a complete HTML and JavaScript example that demonstrates a URIError:
<!DOCTYPE html>
<html>
<head>
<title>URIError Example</title>
</head>
<body>
<h1>URIError Example</h1>
<script>
try {
// Code that might throw a URIError
decodeURIComponent("%xyz"); // Attempting to decode an invalid URI component
} catch (error) {
if (error instanceof URIError) {
// Handle URIError
console.error("URIError occurred:", error.message);
} else {
// Handle other error types
console.error("An error occurred:", error.message);
}
}
</script>
</body>
</html>
In this example:
We have an HTML document with a <script> element containing JavaScript code.
Inside the <script> element, we use a try…catch block to catch potential URIError exceptions.
In the try block, we attempt to decode an invalid URI component using decodeURIComponent(“%xyz”). The URI component “%xyz” is malformed and cannot be decoded, resulting in a URIError.
In the catch block, we check if the caught error is an instance of URIError. If it is, we log an error message specific to URIError. If any other error type is caught, we log a more generic error message.
When you open this HTML file in a web browser and open the browser’s developer console, you will see the error message indicating a URIError due to the attempt to decode an invalid URI component. This demonstrates how to handle a URIError using a try…catch block in JavaScript.
multichoice quiz with answers about this lesson
Here’s a multiple-choice quiz with answers related to error handling in JavaScript:
1-What is the purpose of a try-catch block in JavaScript?
a) To execute code only when there are no errors.
b) To catch and handle exceptions that may occur in the code.
c) To skip specific lines of code.
Answer: b) To catch and handle exceptions that may occur in the code.
2-Which error type is thrown when you try to access a variable that is not defined?
a) SyntaxError
b) TypeError
c) ReferenceError
Answer: c) ReferenceError
3-What is the purpose of the finally block in a try-catch-finally structure?
a) To handle errors that occur in the try block.
b) To execute code regardless of whether an error occurred or not.
c) To execute code only when there are no errors.
Answer: b) To execute code regardless of whether an error occurred or not.
4-Which error type represents issues with the structure of the code, such as missing parentheses or semicolons?
a) SyntaxError
b) TypeError
c) RangeError
Answer: a) SyntaxError
5-What is the primary purpose of using the eval() function in JavaScript?
a) To evaluate arithmetic expressions.
b) To execute code stored as a string.
c) To create custom error messages.
Answer: b) To execute code stored as a string.
6-Which error type is typically thrown when a numeric value is outside the valid range?
a) SyntaxError
b) TypeError
c) RangeError
Answer: c) RangeError
7-What does HTML validation primarily check for?
a) The correctness of JavaScript code.
b) The layout and styling of a web page.
c) The adherence of HTML markup to standards and rules.
Answer: c) The adherence of HTML markup to standards and rules.
8-What is the recommended practice for handling errors in a production JavaScript application?
a) Use only client-side validation.
b) Rely solely on try-catch blocks.
c) Implement both client-side and server-side validation.
Answer: c) Implement both client-side and server-side validation.
9-Which property of the Error object provides information about the error message?
a) name
b) stack
c) message
Answer: c) message
10-When handling errors in JavaScript, what should you consider in addition to client-side validation?
a) Nothing; client-side validation is sufficient.
b) Server-side validation for security and reliability.
c) Using only try-catch blocks for all validation.
Answer: b) Server-side validation for security and reliability.
11-Which of the following statements is true about the catch block in a try-catch structure?
a) The catch block is optional.
b) The catch block is always executed even if there is no error.
c) The catch block is executed only when an error occurs.
Answer: c) The catch block is executed only when an error occurs.
12-In JavaScript, which error type is typically thrown when trying to access a property or method of an undefined or null object?
a) TypeError
b) ReferenceError
c) SyntaxError
Answer: a) TypeError
13-What is the purpose of the throw statement in JavaScript?
a) To catch and handle errors.
b) To explicitly throw an error or custom exception.
c) To terminate the execution of the program.
Answer: b) To explicitly throw an error or custom exception.
14-In a try-catch block, what should you avoid when handling exceptions?
a) Using the throw statement within the catch block.
b) Logging error details for debugging purposes.
c) Swallowing exceptions without proper handling.
Answer: c) Swallowing exceptions without proper handling.
15-What is the purpose of the TypeError error type in JavaScript?
a) To handle issues related to variable references.
b) To represent errors in code syntax.
c) To indicate issues with data types and operations.
Answer: c) To indicate issues with data types and operations.
16-When is server-side validation typically performed in a web application?
a) During client-side form submission.
b) After the user interface is loaded.
c) Before processing user-submitted data on the server.
Answer: c) Before processing user-submitted data on the server.
17-Which HTML element is commonly used to create a form for user input validation?
a) <input>
b) <form>
c) <validation>
Answer: b) <form>
18-In JavaScript, what is the purpose of the finally block when used with try-catch-finally?
a) To catch errors that occur in the finally block.
b) To provide a backup plan for error handling.
c) To execute code regardless of whether an error occurred or not.
Answer: c) To execute code regardless of whether an error occurred or not.
19-Which property of the Error object provides information about the call stack at the point of the error?
a) name
b) stack
c) message
Answer: b) stack
20-Why is it important to perform both client-side and server-side validation in web applications?
a) Client-side validation is faster.
b) Server-side validation is optional.
c) Client-side validation can be bypassed by users, and server-side validation ensures data integrity and security.
Answer: c) Client-side validation can be bypassed by users, and server-side validation ensures data integrity and security.
21-In JavaScript, which error type is thrown when you attempt to access a property or method of an object that does not have that property or method?
a) TypeError
b) ReferenceError
c) SyntaxError
Answer: a) TypeError
22-What is the purpose of the catch block in a try-catch-finally structure?
a) To catch and handle exceptions that may occur in the code.
b) To specify code that will always execute, regardless of errors.
c) To define custom error types.
Answer: a) To catch and handle exceptions that may occur in the code.
23-Which error type in JavaScript is typically thrown when there is a problem with the syntax of the code, such as a missing or misplaced semicolon?
a) SyntaxError
b) TypeError
c) ReferenceError
Answer: a) SyntaxError
24-In HTML validation, what is the primary purpose of using the required attribute for form inputs?
a) To specify that the input is optional.
b) To ensure that the input field is visible on the web page.
c) To indicate that the input field must be filled out before submitting the form.
Answer: c) To indicate that the input field must be filled out before submitting the form.
25-What is the recommended approach for handling errors in a JavaScript application?
a) Avoid using try-catch blocks.
b) Catch all errors and continue execution.
c) Catch specific errors and handle them gracefully.
Answer: c) Catch specific errors and handle them gracefully.
26-In JavaScript, which error type is typically thrown when an operation involves a number that is outside the valid range?
a) SyntaxError
b) TypeError
c) RangeError
Answer: c) RangeError
27-What is the primary purpose of server-side validation in a web application?
a) To enhance user interface design.
b) To improve client-side performance.
c) To ensure the security and integrity of user-submitted data.
Answer: c) To ensure the security and integrity of user-submitted data.
28-Which property of the Error object provides a description of the error message?
a) name
b) stack
c) message
Answer: c) message
29-What does HTML validation primarily check for in web pages?
a) The layout and styling of the web page.
b) The adherence of HTML markup to standards and rules.
c) The presence of JavaScript errors.
Answer: b) The adherence of HTML markup to standards and rules.
30-What is the primary purpose of using the finally block in a try-catch-finally structure in JavaScript?
a) To catch and handle errors.
b) To execute code regardless of whether an error occurred or not.
c) To terminate the program.
Answer: b) To execute code regardless of whether an error occurred or not.