Q119- What are matchers in Jest?

Matchers in Jest are methods used to assert values in your tests. They allow you to check whether a value meets certain conditions and are used with expect().

---

1️⃣ Basic Syntax

expect(value).matcher(expected);

• value → the actual result
• matcher → function to test the value
• expected → the expected outcome

---

2️⃣ Common Matchers

Equality Matchers

expect(2 + 2).toBe(4);          // strict equality (===)
expect({a:1}).toEqual({a:1});   // deep equality for objects/arrays

Truthiness Matchers

expect(null).toBeNull();
expect(undefined).toBeUndefined();
expect(true).toBeTruthy();
expect(false).toBeFalsy();

Number Matchers

expect(10).toBeGreaterThan(5);
expect(10).toBeLessThan(20);
expect(10).toBeGreaterThanOrEqual(10);
expect(10).toBeLessThanOrEqual(10);
expect(0.2 + 0.1).toBeCloseTo(0.3, 5);

String Matchers

expect("Hello World").toMatch(/World/);
expect("abc").toContain("b");

Array/Iterable Matchers

expect([1, 2, 3]).toContain(2);
expect([{a:1}]).toContainEqual({a:1});

Error Matchers

const throwError = () => { throw new Error("Oops"); };
expect(throwError).toThrow("Oops");

---

3️⃣ Negation

• You can negate matchers using .not:

expect(5).not.toBe(3);
expect([1,2,3]).not.toContain(4);

---

⚡ Key Points:

• Matchers are the core of Jest assertions
• Use .toBe for primitives, .toEqual for objects/arrays
• Support numbers, strings, arrays, objects, functions, errors
• Negation (.not) allows opposite assertions

---

In short:
Matchers in Jest are methods to assert conditions in tests, making it easy to verify expected outcomes for variables, functions, and components.