6.4. Error Types

An error type is the classification that JavaScript uses to group errors based on their cause. In future lessons, we will learn that an error type is actually something called a built-in object. For now, understanding the different types of errors will help us become faster at debugging.

Each error that JavaScript reports has an error type, and the type is included in the error message. For example, an earlier message reported the error type as SyntaxError.

/Users/chris/dev/sandbox/js/syntax.js:2
console.log("Hello, name);
            ^^^^^^^^^^^^^^

SyntaxError: Invalid or unexpected token
   at new Script (vm.js:85:7)
   at createScript (vm.js:266:10)
   at Object.runInThisContext (vm.js:314:10)
   at Module._compile (internal/modules/cjs/loader.js:698:28)
   at Object.Module._extensions..js (internal/modules/cjs/loader.js:749:10)
   at Module.load (internal/modules/cjs/loader.js:630:32)
   at tryModuleLoad (internal/modules/cjs/loader.js:570:12)
   at Function.Module._load (internal/modules/cjs/loader.js:562:3)
   at Function.Module.runMain (internal/modules/cjs/loader.js:801:12)
   at internal/main/run_main_module.js:21:11

We have now seen two error types, ReferenceError and SyntaxError. There are several other error types in JavaScript, such as TypeError and RangeError.

The following table describes all JavaScript error types. Some of these relate to coding concepts we have not covered yet, but we include them here as a reference for future use.

JavaScript Error Types
Error Type Description Example of code triggering the error Example description
SyntaxError Occurs when trying to parse syntactically invalid code.
console.log("hello";
The call to console.log does not have a required close parenthesis.
ReferenceError Occurs when a non-existent variable is used/referenced.
1
2
 let firstName = "Jack";
 console.log(firstname);
The variable firstname does not exist; it is a misspelling of firstName.
TypeError Occurs when trying to use a value in an invalid way.
1();
The numeric value 1 is not a function, so trying to use it as one results in TypeError: 1 is not a function.
RangeError Occurs when passing an invalid value to a function.
let nums = Array(-1);
The constructor function Array(n) creates an empty array of length n. It is not possible to create an array with negative length, so the code results in RangeError: Invalid array length.
URIError Occurs when improperly using a global URI-handling function. ('URI' = Uniform Resource Identifier)
decodeURI('%');
The % character is used to encode characters not otherwise allowed in URIs, such as spaces (%20). If an invalid character encoding is given, a URIError results.
Error The type from which all other errors are built. It can be used to generate programmer-triggered and programmer-defined errors.
throw Error("Something bad happened!");
Manually triggers an error with the given message.

Each time you encounter a new error type, take the time to understand what it is, and what JavaScript is trying to tell you. Remember, error messages are your friends!