# expect.addAssertion(...)

Signature:

```js#evaluate:false
expect.addAssertion(pattern, handler);
expect.addAssertion([pattern, ...]], handler);
expect.addAssertion(typeName, pattern, handler);
expect.addAssertion(typeName, [pattern, ...], handler);
expect.addAssertion([typeName, ...], pattern, handler);
expect.addAssertion([typeName, ...], [pattern, ...], handler);
```

New assertions can be added to Unexpected the following way.

```js
var errorMode = 'default'; // use to control the error mode later in the example
expect.addAssertion('array', '[not] to be (sorted|ordered)', function (expect, subject, cmp) {
  expect.errorMode = errorMode;
  expect(subject, '[not] to equal', [].concat(subject).sort(cmp));
});
```

The above assertion definition makes the following expects possible:

```js
expect([1,2,3], 'to be sorted');
expect([1,2,3], 'to be ordered');
expect([2,1,3], 'not to be sorted');
expect([2,1,3], 'not to be ordered');
expect([3,2,1], 'to be sorted', function (x, y) { return y - x; });
```

Let's dissect the different parts of the custom assertion we just
introduced.

The first parameter to `addAssertion` is a string or an array
specifying which types the assertion should be defined on. In this
case the assertion in only defined for arrays. In case the type is not
specified the assertion will be defined for the type `any`, and would
be applicable any type. See the `Extending Unexpected with new types`
section for more information about the type system in Unexpected.

The second parameter to `addAssertion` is a string or an array stating
the patterns this assertion should match. A pattern has the following
syntax. A word in square brackets represents a flag that can either be
there or not. If the flag is present `expect.flags[flag]` will contain
the value `true`. In this case `not` is a flag. When a flag it present
in a nested `expect` it will be inserted if the flag is present;
otherwise it will be removed. Text that is in parentheses with
vertical bars between them are treated as alternative texts that can
be used. In this case you can write _ordered_ as an alternative to
_sorted_.

The last parameter to `addAssertion` is function that will be called
when `expect` is invoked with an expectation matching the type and
pattern of the assertion.

So in this case, when `expect` is called the following way:

```js#evaluate:false
expect([3,2,1], 'to be sorted', reverse);
```

The handler to our assertion will be called with the values the
following way, where the _not_ flag in the nested expect will be
removed:

```js#evaluate:false
expect.addAssertion('array', '[not] to be (sorted|ordered)', function(expect, [3,2,1], reverse){
    expect([3,2,1], '[not] to equal', [].concat([3,2,1]).sort(reverse));
});
```

### Overriding the standard error message

When you create a new assertion Unexpected will generate an error
message from the assertion text and the input arguments. In some cases
it can be preferable to tweak the output instead of creating
completely custom output using [expect.fail](../fail/).

You can override how the `subject` is displayed by providing a
`subjectOutput` for the specific assertion. You can also override the output of
the arguments by overriding parts of `argsOutput` or provide a
completely custom output for the arguments by setting `argsOutput` to
an output function on the assertion.

Here is a few examples:

```js
expect.addAssertion('number', 'to be contained by', function (expect, subject, start, finish) {
    expect.subjectOutput = function (output) {
        output.text('point ').jsNumber(subject);
    };
    expect.argsOutput = function (output) {
        output.text('interval ').text('[').appendInspected(start).text(';').appendInspected(finish).text(']');
    };
    expect(subject >= start && subject <= finish, '[not] to be truthy');
});

expect(4, 'to be contained by', 8, 10);
```

```output
expected point 4 to be contained by interval [8;10]
```

```js
expect.addAssertion('number', 'to be similar to', function (expect, subject, value, epsilon) {
    if (typeof epsilon !== 'number') {
        epsilon = 1e-9;
    }
    expect.argsOutput[2] = function (output) {
        output.text('(epsilon: ').jsNumber(epsilon.toExponential()).text(')')
    }
    expect(Math.abs(subject - value), 'to be less than or equal to', epsilon);
});

expect(4, 'to be similar to', 4.0001);
```

```output
expected 4 to be similar to 4.0001, (epsilon: 1e-9)
```

### Controlling the output of nested expects

When a call to `expect` fails inside your assertion the standard error
message for the custom assertion will be used. In the case of our
_sorted_ assertion fails, the output will be:

```js
expect([ 1, 3, 2, 4 ], 'to be sorted');
```

```output
expected [ 1, 3, 2, 4 ] to be sorted

[
  1,
  3, // should equal 2
  2, // should equal 3
  4
]
```

We can control the output of the nested expects by changing the
`expect.errorMode` property.

The _bubble_ error mode will hoist the next level error message to this level.

If we change the error mode to _bubble_, we get the following output:

```js
errorMode = 'bubble';
expect([ 1, 3, 2, 4 ], 'to be sorted');
```

```output
expected [ 1, 3, 2, 4 ] to equal [ 1, 2, 3, 4 ]

[
  1,
  3, // should equal 2
  2, // should equal 3
  4
]
```

In the _nested_ error mode the next level error message is included and
indented underneath the standard error message.

If we change the error mode to _nested_, we get the following output:

```js
errorMode = 'nested';
expect([ 1, 3, 2, 4 ], 'to be sorted');
```

```output
expected [ 1, 3, 2, 4 ] to be sorted
  expected [ 1, 3, 2, 4 ] to equal [ 1, 2, 3, 4 ]

  [
    1,
    3, // should equal 2
    2, // should equal 3
    4
  ]
```

The _defaultOrNested_ error mode uses the default mode if the error chain contains a diff;
otherwise it the _nested_ error mode will be used.

If we change the error mode to _defaultOrNested_, we get the following output:

```js
errorMode = 'defaultOrNested';
expect([ 1, 3, 2, 4 ], 'to be sorted');
```

```output
expected [ 1, 3, 2, 4 ] to be sorted

[
  1,
  3, // should equal 2
  2, // should equal 3
  4
]
```

In the _diff_ error mode will hoist the first found diff as the error message.
If no diff is present, it will fall back to the _default_ error mode.

If we change the error mode to _diff_, we get the following output:

```js
errorMode = 'diff';
expect([ 1, 3, 2, 4 ], 'to be sorted');
```

```output
[
  1,
  3, // should equal 2
  2, // should equal 3
  4
]
```


### Asynchronous assertions

Unexpected comes with built-in support for asynchronous
assertions. You basically just return a promise from the assertion.

For the purpose of explaining how we can make an asynchronous
assertion we will define a silly type which contains a
value that can only be retrieved after a delay:

```js
function Timelock(value, delay) {
  this.value = value;
  this.delay = delay;
}

Timelock.prototype.getValue = function (cb) {
  var that = this;
  setTimeout(function () {
    cb(that.value);
  }, this.delay);
};
```

It would be pretty nice if we could use
[to satisfy](../../assertions/any/to-satisfy/) on the value of a `Timelock`,
even if the retrieval is delayed. Then we would be able to do stuff
like this:

```js#evaluate:false
return expect(new Timelock('Hello world'), 'to satisfy', expect.it('have length', 11));
```

First we need to define a [type](../addType/) for handling the `Timelock`:

```js
expect.addType({
  name: 'Timelock',
  identify: function (value) {
    return value && value instanceof Timelock;
  },
  inspect: function (value, depth, output) {
    output.jsFunctionName('Timelock');
  }
});
```

```js
expect.addAssertion('Timelock', 'to satisfy', function (expect, subject, spec) {
  return expect.promise(function (run) {
    subject.getValue(run(function (value) {
      return expect(value, 'to satisfy', spec);
    }));
  });
});
```

Let's see how it works:

```js#async:true
return expect(new Timelock('Hello world!', 5), 'to satisfy', expect.it('not to match', /!/));
```

```output
expected Timelock to satisfy expect.it('not to match', /!/)

expected 'Hello world!' not to match /!/

Hello world!
           ^
```

The best resource for learning more about custom assertions is to look
at how the predefined assertions are built:

[lib/assertions.js](https://github.com/unexpectedjs/unexpected/blob/master/lib/assertions.js)