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

VariantJS common functions and utilities

import addMonths from '../../dates/addMonths';

describe('addMonths', () => {
  it('adds 1 month by default', () => {
    const date = new Date(2020, 0, 15);
    const expected = new Date(2020, 1, 15);

    expect(addMonths(date)).toEqual(expected);
  });

  it('adds a month to a date before 1970', () => {
    const date = new Date('0010-02-18T12:00:00.000Z');
    const expected = new Date('0010-03-18T12:00:00.000Z');
    
    expect(addMonths(date)).toEqual(expected);
  });

  it('changes the year if last month', () => {
    const date = new Date(2021, 11, 15);
    const expected = new Date(2022, 0, 15);

    expect(addMonths(date)).toEqual(expected);
  });

  it('uses the last day of next month if doesnt have an equivalent', () => {
    const date = new Date(2021, 9, 31);
    const expected = new Date(2021, 10, 30);

    expect(addMonths(date)).toEqual(expected);
  });

  it('uses the last day of next month if doesnt have an equivalent for a month with 28 days', () => {
    const date = new Date(2021, 0, 31);
    const expected = new Date(2021, 1, 28);

    expect(addMonths(date)).toEqual(expected);
  });

  it('accepts a custom amount of months', () => {
    const date = new Date(2020, 0, 15);
    const expected = new Date(2020, 3, 15);

    expect(addMonths(date, 3)).toEqual(expected);
  });

  it('accepts a negative amount of months', () => {
    const date = new Date(2020, 0, 15);
    const expected = new Date(2019, 9, 15);

    expect(addMonths(date, -3)).toEqual(expected);
  });

  it('doesnt affect the original date', () => {
    const date = new Date(2020, 0, 15);

    expect(date).toEqual(new Date(2020, 0, 15));
  });
});