Neat Little Tricks by Anup Bishnoi

Simple JavaScript assert

It’s better to fail with your own error than undefined is not a function.

A simple assertion solution would let your application fail early and fail at the right point in runtime execution. Allowing you to handle it better.

How you write assertions

For a recent barebones app, I wrote assertions as part of code like this:

function startApplication(url, callback) {
  assert('string' === typeof url,
    'URL needs to be a string!');
  if (callback)
    assert('function' === typeof callback);
  // Application code

If I had needed a slightly more meaty assertion library, I would have used shouldbe with Underscore. But this was just fine for the little app.

Show me the code!

(Or, how you implement simple assertion)

Just throw this simple assert into your application and use it generously:

function assert(condition, message) {
  if (!condition) {
    throw new Error(message ||
      'Assertion failed');