How do you structure a Jest test file?

A readable test file reads top to bottom like a specification. The outer describe names the unit, inner describe blocks name scenarios, and each test states one expected behaviour. Setup should be obvious at the point of use rather than hidden in distant helpers.

describe("applyDiscount", () => {
  describe("when the cart is empty", () => {
    test("returns zero", () => {
      const total = applyDiscount([], 0.1);
      expect(total).toBe(0);
    });
  });

  describe("when a coupon is valid", () => {
    test("subtracts the discount from the subtotal", () => {
      const cart = [{ price: 100 }];
      const total = applyDiscount(cart, 0.1);
      expect(total).toBe(90);
    });
  });
});

The shape of a good case

• Arrange: build the smallest input that exercises the behaviour
• Act: call the unit under test exactly once where possible
• Assert: state the expected outcome with the most specific matcher
• Avoid sharing mutable objects between cases
• Let the test name carry the explanation instead of adding narration

One assertion is not a rule, but one behaviour is. A test may check several related outputs from the same action, such as a returned value and a call to a collaborator. It should not verify three unrelated scenarios, because the first failure hides the rest.