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Throwing your own errors

One of the best ways to manage debugging and errors in JavaScript code is to throw your own errors.

Now, you may be thinking, “How do I reduce errors by causing errors?” The idea is that you throw spe-

cific errors that tell you exactly what error occurred instead of relying on JavaScript’s cryptic

object

expected

error messages. Doing so can cut down debugging time dramatically. Consider a function

designed to divide one number by another:

function divide(iNum1, iNum2) {

return iNum1.valueOf() / iNum2.valueOf();

}

This function assumes several things. First, it assumes that two arguments are passed in; second, it

assumes that both arguments are numbers. But if you make a call that breaks these assumptions, such as

divide(“a”)

, you end up with an error message like

undefined’ is not an object

iNum2 has

no properties

. Add some specific error messages, and the problem becomes clear:

function divide(iNum1, iNum2) {

if (arguments.length != 2) {

throw new Error(“divide() requires two arguments.”);

} else if (typeof iNum1 != “number” || typeof iNum2 != “number”) {

throw new Error(“divide() requires two numbers for arguments.”);

}

return iNum1.valueOf() / iNum2.valueOf();

}

In this case, if you call

divide(“a”)

, you’ll get an error saying

divide() requires two arguments

;

if you call

divide(“a”, “b”)

, you get an error saying

divide() requires two numbers for

arguments

. In both cases, these messages give you much more information than the default JavaScript

error messages and make debugging much easier.

Because of the amount of code required to check for errors and throw messages, many developers create

their own

assert()

function. Many programming languages have an

assert()

method built-in, and

it’s also easy to create your own:

function assert(bCondition, sErrorMessage) {

if (!bCondition) {

throw new Error(sErrorMessage);

}

The

assert()

function simply tests to see if the first argument evaluates to

false

and, if so, throws an

error with the given error message. You can then use

assert()

like this:

function divide(iNum1, iNum2) {

assert(arguments.length == 2, “divide() requires two arguments.”);

assert(typeof iNum1 == “number” && typeof iNum2 == “number”,

“divide() requires two numbers for arguments.”);

return iNum1.valueOf() / iNum2.valueOf();

}

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Error Handling

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