Error handling in JavaScript

31st January 2014 | by Adam Beres-Deak | javascript, browser, error, node.js

Have you ever wondered, what's the proper way of throwing JavaScript errors? And how to handle them? Here we'll see some examples and we'll define some custom error types.

Additional bonus: the following code examples also work in node.js not just in the browsers.

Throwing Errors

Throwing errors is very simple, we just need the throw statement.

function throwsAnError() {
    throw new Error('An error occured');

try {
} catch(ex) {
    console.log(ex.message); // An error occured

Theoretically it's possible to throw any kind of object, but it's not really recommended throwing anything else than Error or one of its derived (custom) types. The reason for this is, that browsers not always work as you would expect. Some of them (IE, Safari) only show "uncaught exception" and don't show the object which was thrown.

Default error types in JavaScript

There are six predefined error types in JavaScript:

Handling specific errors

Here comes the instanceof operator handy, when our codes runs into a catch. With its help we can check which type does the exception belong to.

try {
    // doSomethingWithNumbersAndURIs();
} catch (ex) {
    if (ex instanceof RangeError) {
        alert('Value not in range!');
    } else if (ex instanceof URIError) {
        alert('Value not an URI!');
    } else {
        // some basic error handling

Throwing custom errors

This is also very simple, we just have to create a constructor for our error type which is derived from Error. There's one little catch though, line numbers are shown incorrectly.

function CustomError(message) { = "CustomError";
  this.message = message || "Some default message";
CustomError.prototype = new Error();
CustomError.prototype.constructor = CustomError;

try {
    throw new CustomError('An error occured!');
} catch(ex) {
    console.log(; // CustomError
    console.log(ex.message); // Ann error occured!

If you are interested in more details:

  1. Mozilla Developer Network Documentation
  2. NCZOnline

by Adam Beres-Deak

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