Using "expect" without a matching function (with Supertest and Jest)

Question

I recently ran into the following testing code in an Express app using Supertest and Jest:

const supertest = require("supertest");
const app = require("../app");
const api = supertest(app);

test("notes are returned as json", async () => {
  await api
    .get("/api/notes")
    .expect(200)
    .expect("Content-Type", /application\/json/);
});

I'm kind of confused where the .expect(200) is coming from. Is this part of Supertest? Because I know that in Jest when we call expect we typically use a matcher like this:

expect(200).toBe(200)

But somehow this test works without having to call a matcher.

Answer 1

Supertest v6.3.1

.expect() API comes from the Test class of the supertest package. The key code for handling .expect(200):

// status
if (typeof a === 'number') {
  this._asserts.push(wrapAssertFn(this._assertStatus.bind(this, a)));
  // body 
  // ...
}

The status code will be used like this:

wrapAssertFn(this._assertStatus.bind(this, 200))

See the source code

The this._asserts array holds all the assertion. Supertest will check each assertion until finding a failed assertion here