How do expect and matchers work in Jest?

A matcher is a function that compares the received value with an expected one and throws a descriptive error when the comparison fails. Choosing a precise matcher produces a better failure message than wrapping everything in a boolean check.

const user = { id: 1, name: "Asha", roles: ["admin"] };

expect(2 + 2).toBe(4);
expect(user).toEqual({ id: 1, name: "Asha", roles: ["admin"] });
expect(user).toMatchObject({ name: "Asha" });
expect(user.roles).toContain("admin");
expect("asha@example.com").toMatch(/@example\.com$/);
expect(user.name).toHaveLength(4);
expect(null).toBeNull();
expect(undefined).toBeUndefined();

Matcher families

• Equality: toBe for identity, toEqual for deep equality, toStrictEqual to also check types and undefined keys
• Structure: toMatchObject, toHaveProperty, toContain, toHaveLength
• Truthiness and nullability: toBeTruthy, toBeFalsy, toBeNull, toBeDefined
• Numbers: toBeGreaterThan, toBeCloseTo and domain matchers
• Errors and mocks: toThrow, toHaveBeenCalledWith, toHaveBeenCalledTimes

toBe uses Object.is, so two structurally identical objects fail because they are different references. toEqual ignores undefined properties and does not check class identity, while toStrictEqual is stricter. For a value that should be exactly a plain object, toStrictEqual catches accidental extra keys.