What are runtime errors in JavaScript?

There are three types of errors in programming: (a) Syntax Errors, (b) Runtime Errors, and (c) Logical Errors. Runtime errors, also called exceptions, occur during execution (after compilation/interpretation). For example, the following line causes a runtime error because here the syntax is correct, but at runtime, it is trying to call a method that does not exist.

<script>
   window.printme();  // printme() method doesn't exist
</script>

Uncaught TypeError: window.printme is not a function

Common Types of Runtime Errors

JavaScript throws different types of runtime errors depending on the issue:

ReferenceError

Occurs when trying to access a variable or function that doesn't exist:

console.log(undefinedVariable);

ReferenceError: undefinedVariable is not defined

TypeError

Happens when a value is not of the expected type:

let num = 42;
num.toUpperCase();  // numbers don't have toUpperCase method

TypeError: num.toUpperCase is not a function

RangeError

Occurs when a value is outside the valid range:

let arr = new Array(-1);  // negative array size

RangeError: Invalid array length

Handling Runtime Errors

Use try-catch blocks to handle runtime errors gracefully:

try {
    let result = undefinedVariable;
    console.log(result);
} catch (error) {
    console.log("Caught error:", error.name);
    console.log("Error message:", error.message);
}
console.log("Program continues running");

Caught error: ReferenceError
Error message: undefinedVariable is not defined
Program continues running

Runtime vs Syntax Errors

Exceptions also affect the thread in which they occur, allowing other JavaScript threads to continue normal execution.

Conclusion

Runtime errors occur during code execution when operations fail despite valid syntax. Use try-catch blocks to handle them gracefully and prevent your application from crashing unexpectedly.