Take a look at the source of chai Assertion but the tl;dr is that Chai implements its own chainable methods on its assert library. However, special keywords are simply syntactic sugar as in the code below shows. Literally they are just properties that are added and can be chained but nothing truly is defined:
/**
* ### Language Chains
*
* The following are provided as chainable getters to improve the readability
* of your assertions.
*
* **Chains**
*
* - to
* - be
* - been
* - is
* - that
* - which
* - and
* - has
* - have
* - with
* - at
* - of
* - same
* - but
* - does
*
* @name language chains
* @namespace BDD
* @api public
*/
[ 'to', 'be', 'been'
, 'is', 'and', 'has', 'have'
, 'with', 'that', 'which', 'at'
, 'of', 'same', 'but', 'does' ].forEach(function (chain) {
Assertion.addProperty(chain);
});
From there what it actually looks for are keywords it specifically defines. So one example is the keyword .to.be.true it will look at true as defined in the code snippet below
/**
* ### .true
*
* Asserts that the target is strictly (`===`) equal to `true`.
*
* expect(true).to.be.true;
*
* Add `.not` earlier in the chain to negate `.true`. However, it's often best
* to assert that the target is equal to its expected value, rather than not
* equal to `true`.
*
* expect(false).to.be.false; // Recommended
* expect(false).to.not.be.true; // Not recommended
*
* expect(1).to.equal(1); // Recommended
* expect(1).to.not.be.true; // Not recommended
*
* A custom error message can be given as the second argument to `expect`.
*
* expect(false, 'nooo why fail??').to.be.true;
*
* @name true
* @namespace BDD
* @api public
*/
Assertion.addProperty('true', function () {
this.assert(
true === flag(this, 'object')
, 'expected #{this} to be true'
, 'expected #{this} to be false'
, flag(this, 'negate') ? false : true
);
});
