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Learn JavaScript Syntax: Error Handling

Runtime Error in JavaScript

A JavaScript runtime error is an error that occurs within code while its being executed. Some runtime errors are built-in objects in JavaScript with a name and message property. Any code after a thrown runtime error will not be evaluated.

The throw Keyword in JavaScript

The JavaScript throw keyword is placed before an Error() function call or object in order to construct and raise an error. Once an error has been thrown, the program will stop running and any following code will not be executed.

// The program will raise and output this Error object with message 'Something went wrong'
throw Error('Something went wrong');
//The program will stop running after an error has been raised, and any following code will not be executed.
console.log('This will not be printed');

Javascript Error Function

The JavaScript Error() function creates an error object with a custom message. This function takes a string argument which becomes the value of the error’s message property. An error created with this function will not stop a program from running unless the throw keyword is used to raise the error.

console.log(Error('Your password is too weak.')); //Error: Your password is too weak.

javascript try catch

A JavaScript trycatch statement can anticipate and handle thrown errors (both built-in errors as well as those constructed with Error()) while allowing a program to continue running. Code that may throw an error(s) when executed is written within the try block, and actions for handling these errors are written within the catch block.

// A try...catch statement that throws a constructed Error()
try {
throw Error('This constructed error will be caught');
} catch (e) {
console.log(e); // Prints the thrown Error object
// A try...catch statement that throws a built-in error
const fixedString = 'Cannot be reassigned';
try {
fixedString = 'A new string'; // A TypeError will be thrown
} catch (e) {
console.log('An error occurred!'); // Prints 'An error occurred!'
console.log('Prints after error'); // Program continues running after the error is handled and prints 'Prints after error'