Chapter 3: Matchers With Other Data Types

Truthiness

Sometimes need to distinguish between undefined, null, and false, but you sometimes do not want to treat these differently. Jest contains helpers that let you be explicit about what you want.

• toBeNull matches only null
• toBeUndefined matches only undefined
• toBeDefined is the opposite of toBeUndefined
• toBeTruthy matches anything that an if statement treats as true
• toBeFalsy matches anything that an if statement treats as false

test("null", () => {
  const n = null;
  expect(n).toBeNull();
  expect(n).toBeDefined();
  expect(n).toBeFalsy();
  expect(n).not.toBeUndefined();
  expect(n).not.toBeTruthy();
});

.not

You probably noticed the .not usage above. It make assertion to expect opposite of the matcher.

Strings

You can check strings against regular expressions with toMatch:

test("there is no I in team", () => {
  expect("team").not.toMatch(/I/);
});

test('but there is a "stop" in Christoph', () => {
  expect("Christoph").toMatch(/stop/);
});

Numbers

Most ways of comparing numbers have matcher equivalents.

test("two plus two", () => {
  const value = 2 + 2;
  expect(value).toBeGreaterThan(3);
  expect(value).toBeGreaterThanOrEqual(3.5);
  expect(value).toBeLessThan(5);
  expect(value).toBeLessThanOrEqual(4.5);

  // toBe and toEqual are equivalent for numbers
  expect(value).toBe(4);
  expect(value).toEqual(4);
});

Arrays and Iterables

You can check if an array or iterable contains a particular item using toContain:

const shoppingList = [
  "diapers",
  "kleenex",
  "trash bags",
  "paper towels",
  "milk",
];

test("the shopping list has milk on it", () => {
  expect(shoppingList).toContain("milk");
  expect(new Set(shoppingList)).toContain("milk");
});

Put it to test

We have a utility function and it have different possibilities to return. Check todo.js. It filters todo list with the given filter criteria. I wrote the first test for you. We have a different matcher this time. toHaveLength

1. Create a new test below mine. Make an assertion to check if not-completed tasks length is expected (6).

What is the benefit of this test? If somebody in your team change the filterTodos funtion, lets say he/she used done for completed and undone for not-completed. Our test will fail. We will be able to possible errors before our code goes production ✅