How do assertion libraries such as Chai work without forcing a call to a function?

Question

In Chai, you can do stuff like the following:

expect({}).to.exist;

exist is not a function call, but this still works in testing frameworks. The opposite (expect({}).to.not.exist) causes tests to fail, but again, exist is not a function call.

How do these assertions work without making me call a function? In fact, if I try to say expect({}).to.exist() the test fails because exist is not a function.

Answer 1

I figured it out (or at least, I figured out a method). Use JavaScript getters:

var throws = {
  get a() {
    throw new Error('a');
  },
  get b() {
    throw new Error('b');
  },
  get c() {
    throw new Error('c');
  }
};

When doing throws.a, throws.b, or throws.c, the appropriate error will be thrown.

From that point it is rather easy to build the assertions which are contained within Chai.