arbitrator

Arbitrator
=========

Generative property testing for JavaScript.

arbitrator is a library for generative testing of program properties,
ala QuickCheck.

By providing a specification of the JavaScript program in the form of
properties, the properties can be tested to remain true for a large number of
randomly generated cases. In the case of a test failure, the smallest possible
test case is found.

$3

arbitrator is based on Clojure's test.check
which is inspired by Haskell's QuickCheck.
Many gracious thanks goes to all of the
brilliance and hard work enabling this project to exist.

Note: arbitrator is a fork of testcheck-js.
All credit goes to Lee Byron for getting this thing off the ground.

Getting started
---------------

Install arbitrator using npm

``shell
npm install arbitrator
`

Then require it into your testing environment and start testing.

`javascript
import {Generator, Property} from 'arbitrator';

const result =
Property.forAll(Generator.int, x => x - x === 0).check();
`


API
---

All API documentation is contained within the type definition file, arbitrator.d.ts.


Defining properties
-------------------

A property is simply a function which is expected to always return true, we
might also call these properties "assumptions" or "expectations".

For example, say we wanted to test the assumption that any number subtracted
from itself will be 0, we could define this property as:

`javascript
function (x) {
return x - x === 0;
}
`

Or as another example, let's determine that sorting an array is stable and
idempotent, which is to say that
sorting a sorted array shouldn't do anything. We could write:

`javascript
function (arr) {
var arrCopy = arr.slice();
return deepEqual(arrCopy.sort(), arr.sort().sort());
}
`

That's really it! The only thing special about this property function is that it
is pure, e.g. it relies only on
the provided arguments to determine its return value (no other reading
or writing!).

If you can start to describe your program in terms of its properties, then
arbitrator can test them for you.


Generating test cases
---------------------

Once we've defined some properties, we generate test cases for each properties
by describing the types of values for each argument.

For testing our first property, we need numbers:

`javascript
Generator.int
`

For the second, we need arrays of numbers

`javascript
Generator.int.array()
`

There are a wide variety of value generators, we've only scratched the surface.
We can pick amongst a set of values with
Generator.fromOneOf, nested arrays with ints Generator.int.nested(x => x.array()) and
much more. You can even define your own generators with #map, #chain
and #sized.


Checking the properties
-----------------------

Finally, we check our properties using our test case generator (in this case,
up to 1000 different tests before concluding).

`javascript
const result =
Property.forAll(Generator.int, (x) => x - x === 0)
.check({times: 1000});
`

check runs through random cases looking for failure, and when it doesn't find
any failures, it returns:

`javascript
{ result: true, 'num-tests': 1000, seed: 1406779597155 }
`


Smallest failing test
---------------------

Let's try another property: the sum of two integers is the same or larger than
either of the integers alone.

`javascript
Property.forAll2(Generator.int, Generator.int, (a, b) => {
return a + b >= a && a + b >= b;
}).check();
`

check runs through random cases again. This time it found a failing case, so
it returns:

`javascript
{ result: false,
'failing-size': 2,
'num-tests': 3,
fail: [ 2, -1 ],
shrunk:
{ 'total-nodes-visited': 2,
depth: 1,
result: false,
smallest: [ 0, -1 ] } }
`

Something is wrong. Either:

1. Our assumption is wrong (e.g. bug in our software).
2. The test code is wrong.
3. The generated test data is too broad.

In this case, our problem is that our generated data is too broad for our
assumption. What's going on?

We can see that the fail case 2, -1 would in fact not be correct, but it
might not be immediately clear why. This is where test case shrinking comes in
handy. The shrunk key provides information about the shrinking process and
most importantly, the smallest values that still fail: 0, -1.

We forgot about an edge case! If one of the integers is negative, then the sum
will not be larger. This shrunken test case illustrated this much better than
the original failing test did. Now we know that we can either improve our
property or make the test data more specific:

`javascript
Property.forAll2(Generator.posInt, Generator.posInt, (a, b) => {
return a + b >= a && a + b >= b;
}).check();
`

With our correction, our property passes all tests.


Thinking in random distributions
--------------------------------

It's important to remember that your test is only as good as the data being
provided. While arbitrator provides tools to generate random data, thinking
about what that data looks like may help you write better tests. Also, because
the data generated is random, a test may pass which simply failed to uncover
a corner case.

> "Testing shows the presence, not the absence of bugs"
>
> — Dijkstra, 1969

$3

Visualizing the data check generates may help diagnose the quality of a test.
Use sample to get a look at what a generator produces:

`javascript
Generator.int.sample()
// [ 0, 0, 2, -1, 3, 5, -4, 0, 3, 5 ]
`

$3

Test data generators have an implicit size property, which could be used to
determine the maximum value for a generated integer or the max length of a
generated array. arbitrator begins by generating small test cases and gradually
increases the size.

So if you wish to test very large numbers or extremely long arrays, running
check the default 100 times with maxSize of 200, you may not get what
you expect.

$3

Let's test an assumption that should clearly be wrong: a string split
by another string always returns an array of length 1.

`javascript
Property.forAll2(
Generator.string.notEmpty(),
Generator.string.notEmpty(),
(str, separator) => {
return str.split(separator).length === 1;
}
).check();
`

Unless you got lucky, you probably saw this check pass. This is because we're
testing for a relationship between these strings. If separator is not found
in str, then this test passes. The second random string is very unlikely to
be found within the first random string.

We could change the test to be aware of this relationship such that the
separator is always contained within the str.

`javascript
Property.forAll3(
Generator.string.notEmpty(),
Generator.posInt,
Generator.strictPosInt,
(str, start, length) => {
var separator = str.substr(start % str.length, length);
return str.split(separator).length === 1;
}
).check();
`

Now separator is a random substring of str and the test fails with the
smallest failing arguments: '0', 0, 1`.

Contribution
------------

Use Github issues for requests.

Pull requests actively welcomed. Learn how to contribute.