Exception Handling in JavaScript

What Are Exceptions in JavaScript?

An exception in JavaScript is a disruption of the runtime of a program that - if uncaught - halts that runtime.

ECMAScript & Runtime Environments Exceptions

In JavaScript any primitive and/or object can be used to denote an exception but there are also built-in object constructors to facilitate that. Some of them are declared by the language itself (an ECMAScript exception) or by runtime environments such as browsers (a DOMException). ECMAScript exceptions include such exception constructors as Error, ReferenceError, SyntaxError or TypeError. DOM include such exception constructors as NetworkError, TimeoutError or NotAllowedError. All those exception constructors have message and name properties defined.

foo // Uncaught ReferenceError: foo is not defined

Deliberately Throwing Exceptions

It is possible to deliberately throw an exception in code using the throw keyword.

throw new Error() // Uncaught Error
throw 42 // Uncaught 42
throw '42' // Uncaught 42

Creating Custom Exception Objects

As already noted, any primitive, object or function can be used to denote an exception. Further, it is possible to declare a custom exception object constructor.

function MyCustomException(message) {
  this.name = 'MyCustomException'
  this.message = message

throw new MyCustomException('Something went wrong') // Uncaught MyCustomException {name: "MyCustomException", message: "Something went wrong"}

It is possible to define a property toString to make the exception be pretty printed by various tools - MyCustomException.prototype.toString

Catching Exceptions

Both - built in exceptions and user created exceptions - can be caught and handled which can prevent a program from stopping. An exception in JavaScript is caught using the try...catch statement.

try {
} catch (e) {
} // ReferenceError => undefined

The try...catch statement can be extended with finally. The code in the finally block is executed irrespective of whether an exception was thrown, caught or even handled.

try {
} catch (e) {
} finally {
} // ReferenceError Finally!

Handling Specific Errors

It is possible to handle specific exceptions in the catch block in specific ways. instanceof keyword can be handy at that.

try {
} catch (e) {
  if (e instanceof EvalError) {
    console.log('Not another eval error!')
  } else {
    console.log('Phew! Not an eval error.')
} // Phew! Not an eval error.

Logging Exceptions

When logging exceptions it is advised to use console.error instead of console.log.

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