Jest

Learn the Jest test framework.

What is Jest?

Jest is the test runner that ships with React Native. It provides test() and expect(); you run all tests with npm test. Tests live in the __tests__ folder and use the .test.js suffix.

A test has three parts: a name, a function, and an expected result. test('two plus two', () => { expect(2 + 2).toBe(4); }). expect takes a value; matchers like .toBe compare it. If the match fails Jest shows red. This catches small logic errors early.

test and expect

test(name, fn) defines a test; the first parameter is the test name, the second is the test function. expect(value) takes a value for checking; then matchers like .toBe(value) or .toEqual(value) compare it.

For example expect(result).toBe(4) expects result to equal 4. .toEqual does a deep comparison of objects; .toBe is used for primitives. Multiple expect calls can run in one test; the test stops at the first failure. describe groups related tests; this keeps big test files organized.

Exercise Write your first test

Write a test that checks two plus two equals four. Use test('two plus two', () => { ... }); put expect(2 + 2).toBe(4) inside.

Show example solution

test('two plus two', () => {
  expect(2 + 2).toBe(4);
});

Test yourself

What is Jest's job?
```
test('toplama', () => { expect(1 + 1).toBe(2); });
```
- Compile the app
- Run tests
- Build an APK
- Pick colors
Show answer

Run tests — Jest runs tests and reports the result.

The expect call

test('iki artı iki', () => {____(2 + 2).toBe(4); });

Show answer

test('iki artı iki', () => {expect(2 + 2).toBe(4); });

Match each concept to its role

test

Defines a test
expect

Takes a value
.toBe

Equality check
describe

Groups tests

Next lesson: React Native Testing Library

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