Me: So Guys! Today we will learn JEST. Excited? 😉
You: Yes!😅
Me: Let's start, then...🤭

WHAT IS JEST? 🧐

Jest is a delightful JavaScript Testing Framework with a focus on simplicity. It works with projects using: Babel, TypeScript, Node, React, Angular, Vue and more!

ADVANTAGES OF JEST 😎

1. Fast and safe
2. Code Coverage
3. Easy Mocking
4. Great Exceptions (if tests fail, it provides rich context why)
5. Zero Configuration

INSTALLATION 😇

If you use yarn...

yarn add -D jest

npm fan...

npm install -D jest

Before examples, we will understand some basics global methods or objects of jest.

GLOBALS 🤗

1. describe(name, fn)

This global method of jest creates a block that group together various tests!
For Example,

describe('arithmetic', () => {

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

  test('substract', () => {
    expect(5-2).toBe(3);
  });

} )

Note: describe() is not compulsory, but helps to organise the group of tests together. Note: there is describe.only() if you want to run only one describe block

2. test(name, fn, timeout)

This jest method takes 3 arguments i.e The first argument is the test name; the second argument is a function that contains the expectations to test. The third argument (optional) is a timeout (in milliseconds).
Note: The default timeout is 5 seconds.

test(' Harry can talk to snakes ?  ', () => {
    expect( canHarryTalkToSnakes() ).toBeTruthy()
} )

Please, assume that canHarryTalkToSnakes() is a function defined in some other file that returns true!

Note: If a promise is returned from test(), Jest will wait for the promise to resolve before letting the test complete.

Note: test.only(name, fn, timeout) this will run only a subset of tests which have .only()

Expect 😬

Expect gives us various matchers to test values against certain conditions!

You: Payal, What are matchers?
Me:

1. expect(value)

Expect, is used every time you need to test a value. And expect expects 😂 a "matcher" function with itself always to assert something about a value. 😆 For Example,

test('Help will always be given at Hogwarts to those who ask for it', () => {
  expect( voldemortHasHorcruxes() ).toBe(7)
} )

So here, .toBe() is a "matcher" function

2. expect.extend(matchers)

We can use expect.extend to write our own matcher function

checkout example here

Matchers

Some common matchers are as follows

1. .toBe() - tests against equality
2. .toEqual() - recursively checks every field of an object or array
3. .not().toBe() - opposite of equality
4. .toBeNull() - matches only null
5. .toBeUndefined() - matches only undefined
6. .toBeTruthy() - if(true)
7. .toBeFalsy() - if(false)
8. .toContain() - if an array or iterable contains a particular item
9. .toMatch() - check strings against regular expressions
10. .toBeGreaterThan() - greater than value
11. .toBeGreaterThanOrEqual() - greater than and equal to equal

Note: same goes with toBeLessThan and toBeLessThanOrEqual

You: 😮

CONGRATULATIONS 😍

Here we are done, I will cover asynchronous tests in my next blog! Thank you guys for reading up till here, I hope you enjoyed and learnt something new!
See you! 👋