BEFOREEACH.md

The need to use beforeEach

let's say we test addOne that adds 1 to the number it receives:

const addOne = (value: number): number => value + 1;

test failing in jest

You could write your tests like this:

import { addOne } from './addOne';

describe('addOne Fail', () => {
  let param: number;

  describe('when param = 0', () => {
    param = 0;

    it('should return 1', () => {
      expect(addOne(param)).toEqual(1);
    });
  });

  describe('when param = 1', () => {
    param = 1;

    it('should return 2', () => {
      expect(addOne(param)).toEqual(2);
    });
  });
});

You expect jest to run this test like this:

import { addOne } from './addOne';
let param: number;
param = 0;
expect(addOne(param)).toEqual(1);
param = 1;
expect(addOne(param)).toEqual(2);

But it actually runs it like this:

import { addOne } from './addOne';
let param: number;
param = 0;
param = 1;
expect(addOne(param)).toEqual(1);
expect(addOne(param)).toEqual(2);

which results in the first test failing.

test succeeding in jest

instead, you have to write it using beforeEach:

import { myFunction } from './myFunction';

describe('addOne success', () => {
  let param: number;

  describe('when param = 0', () => {
    beforeEach(() => {
      param = 0;
    });

    it('should return 1', () => {
      expect(addOne(param)).toEqual(1);
    });
  });

  describe('when param = 1', () => {
    beforeEach(() => {
      param = 1;
    });

    it('should return 2', () => {
      expect(addOne(param)).toEqual(2);
    });
  });
});

Which results in jest to run this test like this, which succeeds:

import { addOne } from './addOne';
let param: number;
param = 0;
expect(addOne(param)).toEqual(1);
param = 1;
expect(addOne(param)).toEqual(2);

using check

using check

• each test is run in its own new context (no variable leak between tests),
• all the code you pass through from the top of the file to reach that test branch is executed,

so using the first example above,

import { addOne } from './addOne';

describe('addOne Fail', () => {
  let param: number;

  describe('when param = 0', () => {
    param = 0;

    it('should return 1', () => {
      expect(addOne(param)).toEqual(1);
    });
  });

  describe('when param = 1', () => {
    param = 1;

    it('should return 2', () => {
      expect(addOne(param)).toEqual(2);
    });
  });
});

those tests would run like this:

import { addOne } from './addOne';
let param: number;
param = 0;
expect(addOne(param)).toEqual(1);
// initializing new context
import { addOne } from './addOne';
let param: number;
param = 1;
expect(addOne(param)).toEqual(2);