jankycheck

Do not use this

At least don't use it for anything real. I make no promises about it working well. I mean, really, didn't you see the name?

If you must try it out

Install

$ npm install -g git://github.com/eventualbuddha/jankycheck.git

You don't have to install it globally, it's just easier that way.

Write some properties

Create a module with export whose names start with prop_. These should be functions that take however many random arguments you need and return a boolean to indicate success or failure. If the function takes arguments it should have a generators property containing an array of gentest generators. Here's an example:

// myprops.js
var gentest = require('gentest');

function add(x,y) {
  return x + y;
}

function prop_isCommutative(x, y) {
  return add(x, y) === add(y, x);
}

prop_isCommutative.generators = [gentest.types.int, gentest.types.int];
exports.prop_isCommutative = prop_isCommutative;

$ jankycheck myprops.js
✓ prop_isCommutative

1 total, 1 passed, 0 failed.

PDD (Property-Driven Development)

jankycheck uses gentest to handle the generation of random data to pass to properties under test. It also uses it to check the properties. jankytest intercepts failures and automatically shrinks the failing test case down to a minimal failing test using shrinker. Given this broken pow function and property:

var gentest = require('gentest');

function pow(base, exponent) {
  var result = 1;
  while (--exponent > 0) {
    result *= base;
  }
  return result;
}

function prop_powWorks(base, exponent) {
  return pow(base, exponent) === Math.pow(base, exponent);
}
prop_powWorks.generators = [gentest.types.int, gentest.types.int];
exports.prop_powWorks = prop_powWorks;

pow looks like it might work, but let's see what happens when we try it:

$ jankycheck myprops.js
  ✘ prop_powWorks

    Failed with test case: prop_powWorks(-1, -1)
    Shrunk after 2 iterations to prop_powWorks(0, 1)


  1 total, 0 passed, 1 failed.

We aren't handling a base of 0 correctly or negative exponents. Let's fix base of 0 first.

function pow(base, exponent) {
  if (base === 0) {
    return 0;
  }

  var result = 1;
  while (--exponent > 0) {
    result *= base;
  }
  return result;
}

Looks better! Let's try again.

$ jankycheck myprops.js
  ✘ prop_powWorks

    Failed with test case: prop_powWorks(0, -1)
    Shrunk after 1 iteration to prop_powWorks(0, 0)


  1 total, 0 passed, 1 failed.

Oh right, 00 is 1, not 0.

function pow(base, exponent) {
  if (exponent === 0) {
    return 1;
  }

  if (base === 0) {
    return 0;
  }

  var result = 1;
  while (--exponent > 0) {
    result *= base;
  }
  return result;
}

Okay, that should fix the zero exponent case.

$ jankycheck myprops.js
✘ prop_powWorks

  Failed with test case: prop_powWorks(-3, 3)
  Shrunk after 4 iterations to prop_powWorks(2, 1)


1 total, 0 passed, 1 failed.

Grr, something else is busted. Maybe --exponent should be exponent--?

function pow(base, exponent) {
  if (exponent === 0) {
    return 1;
  }

  if (base === 0) {
    return 0;
  }

  var result = 1;
  while (exponent-- > 0) {
    result *= base;
  }
  return result;
}

Let's see what that did…

$ jankycheck myprops.js
✘ prop_powWorks

  Failed with test case: prop_powWorks(4, -2)
  Shrunk after 2 iterations to prop_powWorks(0, -1)


1 total, 0 passed, 1 failed.

That's the problem we saw before with negative exponents! After fixing this we run into an issue where our results are not the same as Math.pow. This is probably attributable to floating point math weirdness. Also, what happens if base is 0 and exponent is less than 0? Division by zero! Turns out that's okay in JS as you just get Infinity. tl;dr, here's the final pow function and property:

var gentest = require('gentest');

function pow(base, exponent) {
  if (exponent < 0) {
    return pow(1/base, -exponent);
  }

  if (exponent === 0) {
    return 1;
  }

  if (base === 0) {
    return 0;
  }

  var result = 1;
  while (exponent-- > 0) {
    result *= base;
  }
  return result;
}

function prop_powWorks(base, exponent) {
  var result = pow(base, exponent);
  var reference = Math.pow(base, exponent);
  return result === reference || (Math.abs(result - reference) / reference) < 0.00000001;
}
prop_powWorks.generators = [gentest.types.int, gentest.types.int];
exports.prop_powWorks = prop_powWorks;

Let's run it just to make sure:

$ jankycheck myprops.js
✓ prop_powWorks

1 total, 1 passed, 0 failed.

The great thing about this was that we didn't have to keep thinking of test cases. Instead, they were just handed to us (in minimal form) and all we had to do was figure out why they failed. We did have to modify our property a little bit (yay floats!), but not much.

You're right that this is a contrived example since we had a reference implementation we could check our result against. But often you'll have a slow-but-working implementation and a fast-but-untested implementation that you can compare against each other. Also, you can do model-based property testing where you model the system under test using simple primitives you know work (such as using the built-in Array class to test your custom Set class).

Contributing

Well, probably don't. If you must, create a branch and submit a PR.

License

MIT, but you're not using this library for real, right?