How does the expect().to.be.true work in Chai?

Question

expect(true).to.be.true;

In this code, all the 'to', 'be', 'true' seems to be an attribute of the object response from 'expect(true)'.

How can these attributes work so that they can raise an exception?

Answer 1

You can check the source code:

[ 'to', 'be', 'been'
  , 'is', 'and', 'has', 'have'
  , 'with', 'that', 'which', 'at'
  , 'of', 'same', 'but', 'does' ].forEach(function (chain) {
    Assertion.addProperty(chain);
});

and there is a addProperty in utils:
https://github.com/chaijs/chai/blob/master/lib/chai/utils/addProperty.js

With this you can chain the properties infinitely like: .to.be.to.to.to.be.equal()

Let's create a simpler demonstration:

Assume that you have an assert object, with the .true() method

const assert = {
  'true': function (v) {
    return !!v
  }
}

and you want to be able to chain .to infinitely. Simply Use the defineProperty to define our getter:

Object.defineProperty(assert, 'to', {
  get() {
    return assert
  }
})

so now you can

assert.to.to.to.to.true(false)

working code: https://codepen.io/CodinCat/pen/LLzBEX?editors=0012

---

I've added another more complex example here: https://codepen.io/CodinCat/pen/dRVjXL?editors=0012

In this example you can see that there is some behaviors in the .true property.

We store the value from expect() in the internal __expectObj and the __value property, and then verify it in the getter of .true. So you can

expect(false).to.to.to.to.true