Building a Helper

This tutorial assumes that you are familiar with the plugin core concepts. If you have not yet read that article, it is recommended that you do so before continuing.

Providing chainable helper assertions is the most common use of the plugin utilities that Chai exposes. Before we get into the basics, we are going to need a topic for which we will extend Chai’s assertions to comprehend. For this, we will be using a very minimal data model object.

/**
 * # Model
 *
 * A constructor for a simple data model
 * object. Has a `type` and contains arbitrary
 * attributes.
 *
 * @param {String} type
 */

function Model (type) {
  this._type = type;
  this._attrs = {};
}

/**
 * .set (key, value)
 *
 * Set an attribute to be stored in this model.
 *
 * @param {String} key
 * @param {Mixted} value
 */

Model.prototype.set = function (key, value) {
  this._attrs[key] = value;
};

/**
 * .get (key)
 *
 * Get an attribute that is stored in this model.
 *
 * @param {String} key
 */

Model.prototype.get = function (key) {
  return this._attrs[key];
};

Practically speaking, this could be any data model object returned from an ORM database in node or constructed from your MVC framework of choice in the browser.

Hopefully our Model class is self explanatory, but as an example, here we construct a person object.

var arthur = new Model('person');
arthur.set('name', 'Arthur Dent');
arthur.set('occupation', 'traveller');
console.log(arthur.get('name')); // Arthur Dent

Now that we have our subject, we can move on to the basics of plugins.

Adding Language Chains

Now we are getting to the fun part! Adding properties and methods are what Chai’s plugin API is really for.

Adding Properties

In essence, defining a property can be done using Object.defineProperty, but we encourage you to use Chai’s utility helpers to ensure a standard implementation throughout.

For this example, we want the following test case to pass:

var arthur = new Model('person');
expect(arthur).to.be.a.model;

For this, we will use the addProperty utility.

utils.addProperty(Assertion.prototype, 'model', function () {
  this.assert(
      this._obj instanceof Model
    , 'expected #{this} to be a Model'
    , 'expected #{this} to not be a Model'
  );
});

View addProperty API

Simple and concise. Chai can take it from here. It is also worth mentioning that because this extension pattern is used so often, Chai makes it just a bit easier. The following can be used in place of the first line above:

Assertion.addProperty('model', function () { // ...

All chain extension utilities are provided both as part of the utils object and directly on the Assertion constructor. For the rest of this document, however, we will be calling the methods directly from Assertion.

Adding Methods

Note: Multiple plugins defining the same method name using addMethod will conflict, with the last-registered plugin winning. The plugin API is pending a major overhaul in future versions of Chai that will, among other things, deal with this conflict. In the mean time, please prefer using overwriteMethod.

Though a property is an elegant solution, it is likely not specific enough for the helper we are constructing. As our models have types, it would be beneficial to assert that our model is of a specific type. For this, we need a method.

// goal
expect(arthur).to.be.a.model('person');

// language chain method
Assertion.addMethod('model', function (type) {
  var obj = this._obj;

  // first, our instanceof check, shortcut
  new Assertion(this._obj).to.be.instanceof(Model);

  // second, our type check
  this.assert(
      obj._type === type
    , "expected #{this} to be of type #{exp} but got #{act}"
    , "expected #{this} to not be of type #{act}"
    , type        // expected
    , obj._type   // actual
  );
});

View addMethod API

All calls to assert are synchronous, so if the first one fails the AssertionError is thrown and the second one will not be reached. It is up to the test runner to interpret the message and handle display of any failed assertions.

Methods as Properties

Chai includes a unique utility that allows you to construct a language chain that can function as either a property or a method. We call these “chainable methods”. Despite the fact that we demonstrated the “is model of model” as both a property and a method, these assertions are NOT a good use case for chainable methods.

When to Use

To understand when to best use chainable methods we will examine a chainable method from Chai’s core.

var arr = [ 1, 2, 3 ]
  , obj = { a: 1, b: 2 };

expect(arr).to.contain(2);
expect(obj).to.contain.key('a');

For this to work, two separate functions are needed. One that will be invoked when the chain is used as either a property or a method, and one that will be invoked when only used as a method.

In these examples, and with all of the other chainable methods in core, the only function of contain as a property is to set a contains flag to true. This indicates to keys to behave differently. In this case, when key is used in conjunction with contain, it will check for the inclusion of a key, instead of checking the exact match to all keys.

When NOT to Use

Let’s say we set up a chainable method for model to behave as we indicated above: do an instanceof check if used as a property, and a _type check if used as a method. The following conflict would occur…

The following would work…

expect(arthur).to.be.a.model;
expect(arthur).to.be.a.model('person');
expect(arr).to.not.be.a.model;

But the following would not…

expect(arthur).to.not.be.a.model('person');

Remember, since the function used as the property assertion is invoked when also used as a method, and negation impacts ALL assertions after it is set, we would receive an error message resembling expected [object Model] not to be instance of [object Model]. As such, please obey this general guideline when constructing chainable methods.

When constructing chainable methods, the property function should only serve to set a flag for later modifying the behavior of an existing assertion.

An Appropriate Example

For use with our model example, we are going to construct an example that allows us to test Arthur’s age exactly, or chain into Chai’s numerical comparators, such as above, below, and within. You will need to learn how to overwrite methods without destroying core functionality, but we get to that a bit later.

Our goal will allow for all of the following to pass.

expect(arthur).to.have.age(27);
expect(arthur).to.have.age.above(17);
expect(arthur).to.not.have.age.below(18);

Let’s start first by composing the two functions needed for a chainable method. First up is the function to use when invoking the age method.

function assertModelAge (n) {
  // make sure we are working with a model
  new Assertion(this._obj).to.be.instanceof(Model);

  // make sure we have an age and its a number
  var age = this._obj.get('age');
  new Assertion(age).to.be.a('number');

  // do our comparison
  this.assert(
      age === n
    , "expected #{this} to have have #{exp} but got #{act}"
    , "expected #{this} to not have age #{act}"
    , n
    , age
  );
}

By now, that should be self-explanatory. Now for our property function.

function chainModelAge () {
  utils.flag(this, 'model.age', true);
}

We will later teach our numerical comparators to look for that flag and change its behavior. Since we don’t want to break the core methods, we will need to safely override that method, but we’ll get to that in a minute. Let’s finish up here first…

Assertion.addChainableMethod('age', assertModelAge, chainModelAge);

View addChainableMethod API

Done. Now we can assert Arthur’s exact age. We will pick up again with this example when learning how to overwrite methods.

Overwriting Language Chains

Now that we can successfully add assertions to the language chain, we should work on being able to safely overwrite existing assertions, such as those from Chai’s core or other plugins.

Chai provides a number of utilities that allow you to overwrite existing behavior of an already existing assertion, but revert to the already defined assertion behavior if the subject of the assertion does not meet your criteria.

Let’s start with a simple example of overwriting a property.

Overwriting Properties

For this example, we are going to overwrite the ok property provided by Chai’s core. The default behavior is that ok will pass if an object is truthy. We want to change that behavior so then when ok is used with an instance of a model, it validates that the model is well formed. In our example, we will consider a model ok if it has an id attribute.

Let’s start out with the basic overwrite utility and a basic assertion.

chai.overwriteProperty('ok', function (_super) {
  return function checkModel () {
    var obj = this._obj;
    if (obj && obj instanceof Model) {
      new Assertion(obj).to.have.deep.property('_attrs.id').a('number');
    } else {
      _super.call(this);
    }
  };
});

View overwriteProperty API

Overwrite Structure

As you can see, the main difference in overwriting is that the first function passes just one argument of _super. This is the function that originally existed, and you should be sure to call that if your criteria doesn’t match. Secondly, you will notice that we immediately return a new function that will serve as the actual assertion.

With this in place, we can write positive assertions.

var arthur = new Model('person');
arthur.set('id', 42);
expect(arthur).to.be.ok;
expect(true).to.be.ok;

The above expectations will pass. When working with a model it will run our custom assertion, and when working with non-models it will revert to the original behavior. We will, however, run into a bit of trouble if we try to negate an ok assertion on a model.

var arthur = new Model('person');
arthur.set('id', 'dont panic');
expect(arthur).to.not.be.ok;

We would expect this expectation to pass as well, as our statement is negated and the id is not a number. Unfortunately, the negation flag was not passed to our number assertion, so it still expects the value to be a number.

Transferring Flags

For this we will expand on this assertion by transferring all of the flags from the original assertion to our new assertion. The final property overwrite would look like this.

chai.overwriteProperty('ok', function (_super) {
  return function checkModel () {
    var obj = this._obj;
    if (obj && obj instanceof Model) {
      new Assertion(obj).to.have.deep.property('_attrs.id'); // we always want this
      var assertId = new Assertion(obj._attrs.id);
      utils.transferFlags(this, assertId, false); // false means don't transfer `object` flag
      assertId.is.a('number');
    } else {
      _super.call(this);
    }
  };
});

Now, the negation flag is included in your new assertion and we can successfully handle both positive and negative assertions on the type of id. We left the property assertion as it was as we always want it to fail if the id is not present.

Enhancing Error Messages

Though, we have one more small modification to make. Should our assertion fail for the wrong type of id attribute, we would get an error message that states expected 'dont panic' to [not] be a number. Not entirely useful when running a large test suite, so we will provide it with a bit more information.

var assertId = new Assertion(obj._attrs.id, 'model assert ok id type');

This will change our error message to be a more informative model assert ok id type: expected 'dont panic' to [not] be a number. Much more informative!

Overwriting Methods

Overwriting methods follow the same structure of overwriting properties. For this example we will be returning to our example of asserting Arthur’s age to be above a minimum threshold.

var arthur = new Model('person');
arthur.set('age', 27);
expect(arthur).to.have.age.above(17);

We already have our age chain in place to flag the assertion with model.age so all we have to do is check if that exists.

Assertion.overwriteMethod('above', function (_super) {
  return function assertAge (n) {
    if (utils.flag(this, 'model.age')) {
      var obj = this._obj;

      // first we assert we are actually working with a model
      new Assertion(obj).instanceof(Model);

      // next, make sure we have an age
      new Assertion(obj).to.have.deep.property('_attrs.age').a('number');

      // now we compare
      var age = obj.get('age');
      this.assert(
          age > n
        , "expected #{this} to have an age above #{exp} but got #{act}"
        , "expected #{this} to not have an age above #{exp} but got #{act}"
        , n
        , age
      );
    } else {
      _super.apply(this, arguments);
    }
  };
});

View overwriteMethod API

This covers both positive and negative scenarios. No need to transfer flags in this case as this.assert handles that automatically. The same pattern can also be used for below and within.