Throwing errors is hard. Bad errors give you useless information and mislead you. Good errors help you debug your code and suggest possible fixes. I highly recommend you to read a post by Nicholas Zakas on this subject: The Art of Throwing JavaScript Errors Part 1 (why throw and when to throw) and Part 2 (what to throw). Today I would like to discuss practical part of throwing errors.

JavaScript is a very flexible language. As my experience grows my code tend to be more elegant and concise. I try to express more in less. I often use functional programming concepts like currying (see also Why Ramda?) and shortcuts of conditionals with boolean operators (&& and ||, to get more sense about using JavaScript logical operators as conditionals check out this article by Addy Osmani).

I used to read and write code like this:

var port = process.env['PORT'] || 3000;
DEBUG && console.log('Port is', port);

But how do we usually throw errors? For example, we might want to check that required environment variable is set:

var port = process.env['PORT'];

if (!port) {
    throw new Error('PORT is not specified');
}

As you may know, JavaScript have expressions and statements. Axel Rauschmayer in Expressions vs Statements tells us:

Wherever JavaScript expects a statement, you can also write an expression. […] The reverse does not hold: you cannot write a statement where JavaScript expects an expression.

One of the disadvantages of throw is that it is not an expression and you can’t use it in expression context:

var port = process.env['PORT'] || throw new Error('PORT is not specified');
// SyntaxError: Unexpected token throw

That bothered me so much that I decided to just wrap throw in a function (so that it becomes an expression) and drop it into npm module (on GitHub) to reuse in my projects:

var thr = require('throw');

var port = process.env['PORT'] || thr('PORT is not specified');

It is very simple module yet extremely useful. It also utilizes printf-like arguments and you can get rid of string concatenation when constructing error messages:

function getEnvVar(variableName) {
    return process.env[variableName] || thr('Environment variable "%s" is required', variableName);
}

var parsed = parse(input) || thr('Could not parse', input);

function firstMatch(regex, input) {
    var match = regex.exec(input) || thr('%s does not match %s', input, regex);
    return match[1];
}

Maybe it help you too.

Currently custom error types are not supported in node-throw, but they may eventually (file an issue with use case or just send me a pull request).

Some additional links:

• Exceptions: Why throw early? Why catch late?

Have a nice day!