17.1. Introduction

Errors are a part of coding. Occasionally, we make mistakes as programmers. However, we are always trying to fix those mistakes by reading different resources, examining a list of error messages (also called the stacktrace), or asking for help.

Earlier in this course, we learned about two different types of errors: runtime and logic. A logic error is when your program executes without breaking, but doesn't behave the way you thought it would. These logic errors usually require you to consider how you are going about solving the issue to resolve. Runtime errors are when your program does not run correctly, and an exception is raised.

An exception is a runtime error in which a name and message are displayed to provide more information about the error.

17.1.1. Exceptions and Errors

In JavaScript a runtime error and an exception are the same thing and can be used interchangeably. This can cause confusion because a logic error is not an exception!

17.1.2. Error Object

When a runtime error, also known as an exception, is raised JavaScript returns an Error object. An Error Object has two properties: a name and a message. The name refers to the type of error that occurred, while the message gives the user information on why that exception occurred.

JavaScript has built-in exceptions with pre-defined names and messages, however, JavaScript also gives you the ability to create your own error messages.

You have undoubtedly experienced various Exceptions already throughout this class. Let's look at a few common Exceptions.

17.1.3. Common Exceptions

JavaScript has some built-in Exceptions you may have already encountered in this class.

One of the most common errors in JavaScript is a SyntaxError which is thrown when we include a symbol JavaScript is not expecting.

Example

console.log("This is" an example);

Console Output

SyntaxError: missing ) after argument list

We put our second quotation mark in the incorrect place. JavaScript does not know what to do with the second half of our phrase and throws a SyntaxError with the message: missing ) after argument list.

A ReferenceError is thrown when we try to use a variable that has not yet been defined.

Example

console.log(x[0]);

Console Output

ReferenceError: x is not defined

We attempt to print out the first element in the variable x, but we never declared x. JavaScript throws a ReferenceError with the message: x is not defined.

A TypeError is thrown when JavaScript expects something to be one type, but the provided value is a different type.

Example

const a = "Launch";

a = "Code";

Console Output

TypeError: invalid assignment to const 'a'

In this case, we declare a constant as the string "Launch", and then try to change the immutable variable to "Code". JavaScript throws a TypeError with the message: invalid assignment to const 'a'.

Exceptions give us a way to provide more information on how something went wrong. JavaScript's built-in Exceptions are regularly used in the debugging process.

There are more built-in Exceptions in JavaScript, you can read more by referencing the MDN Errors Documentation or W3Schools JavaScript Error (scroll down to the Error Object section).

In the next section we will learn how to raise our own exceptions using the throw statement.

17.1.4. Check Your Understanding

Question

What is the difference between a runtime error, and a logic error?

Question

What are some of the common errors included in JavaScript?