@variantjs/core/src/__tests__/dates/getFirstDayOfPrevMonth.test.ts

VariantJS common functions and utilities

import getFirstDayOfPrevMonth from '../../dates/getFirstDayOfPrevMonth';

describe('getFirstDayOfPrevMonth', () => {
  it('gets the first day of prev month if is first month', () => {
    // 2020-01-15
    const date = new Date(2020, 0, 15);

    expect(getFirstDayOfPrevMonth(date)).toEqual(new Date(2019, 11, 1));
  });

  it('gets the first day of prev month if is last month', () => {
    // 2020-01-15
    const date = new Date(2020, 11, 15);

    expect(getFirstDayOfPrevMonth(date)).toEqual(new Date(2020, 10, 1));
  });

  it('gets the first day of prev month when first day', () => {
    // 2020-01-1
    const date = new Date(2020, 11, 1);

    expect(getFirstDayOfPrevMonth(date)).toEqual(new Date(2020, 10, 1));
  });

  it('doesnt affects the original date', () => {
    const originalDate = new Date(2020, 0, 15, 10, 11, 12);
    const date = new Date(2020, 0, 15, 10, 11, 12);

    getFirstDayOfPrevMonth(date);

    expect(date).toEqual(originalDate);
  });

  it('works with dates before 1970', () => {
    const date = new Date('0010-02-18T00:00:00.000Z');
    const expected = new Date('0010-01-01T00:00:00.000Z');
    
    expect(getFirstDayOfPrevMonth(date)).toEqual(expected);
  });
});