JavaScript runtime errors occur when the code is being executed and something unexpected happens that prevents the code from running properly. Here are some common runtime errors in JavaScript:
- TypeError: Occurs If performing an operation on a value that is not of the expected type. For example, trying to call a method on a null or undefined value.
- ReferenceError: When you try to reference a variable or function that has not been declared. For example, trying to use a variable that has not been defined.
- SyntaxError: This error occurs when there is a syntax error in your code. For example, a missing semicolon or a missing closing parenthesis.
- RangeError: If you try to use a value that is outside the range of valid values. For example, trying to create an array with a negative length.
- EvalError: This error occurs when there is an error in the eval() function. This function is used to evaluate a string as JavaScript code.
JavaScript Runtime errors example
Simple example code using a try-catch block to handle a runtime error in JavaScript:
<!DOCTYPE html>
<html>
<body>
<script>
try {
// This code might throw a runtime error
let value = null;
let length = value.length;
console.log(length);
} catch (error) {
console.error('Runtime error:', error.message);
let length = 0;
console.log(length);
}
</script>
</body>
</html>Output:
Here’s another example:
try {
// This code might throw a runtime error
let value = JSON.parse('{"name":"John"}');
console.log(value.age);
} catch (error) {
// If a runtime error occurs, handle it here
if (error instanceof TypeError) {
console.error('Type error:', error.message);
// Provide a fallback value or solution
let value = { name: "John", age: null };
console.log(value.age);
} else {
console.error('Runtime error:', error.message);
}
}
Do comment if you have any doubts on this Js error topic.
Note: The All JS Examples codes are tested on the Firefox browser and the Chrome browser.
OS: Windows 10
Code: HTML 5 Version