latest change helper

latest_change.js

export function latestChange(...firstArgs) {
  let oldState = firstArgs;
  let prevValue = null;

  return (...args) => {
    let found = false;

    const newState = oldState.map((oldVal, i) => {
      const val = args[i];
      if (!found && oldVal !== val) {
        found = true;
        prevValue = val;
      }
      return val;
    });

    oldState = newState;

    return prevValue;
  };
}

tests.js

// jest tests

import { latestChange } from '../latest_change';

describe('latestChange', () => {
  it('returns a function', () => {
    const checker = latestChange();
    expect(typeof checker).toBe('function');
  });

  it('returns null without args', () => {
    const checker = latestChange();
    expect(checker()).toBe(null);
  });

  describe('checker function', () => {
    let checker;

    beforeEach(() => {
      checker = latestChange(1, 2, 3);
    });

    it('returns null if nothing changed', () => {
      expect(checker(1, 2, 3)).toBe(null);
    });

    it('returns the latest value', () => {
      expect(checker(1, 4, 3)).toBe(4);
    });

    it('returns the newst value every time', () => {
      expect(checker(1, 4, 3)).toBe(4);
      expect(checker(10, 4, 3)).toBe(10);
      expect(checker(10, 4, 30)).toBe(30);
    });

    it('returns the previous value if nothing changed', () => {
      expect(checker(1, 4, 3)).toBe(4);
      expect(checker(1, 4, 3)).toBe(4);
    });

    it('returns only the first changed value', () => {
      expect(checker(2, 4, 3)).toBe(2);
      expect(checker(2, 10, 11)).toBe(10);
    });

    it('does not check new arguments', () => {
      // 4th arg is new, so nothing changed compared to the first state
      expect(checker(1, 2, 3, 4)).toBe(null);
      expect(checker(1, 2, 3, 5)).toBe(null);
      expect(checker(1, 2, 3, 6)).toBe(null);
    });

    it('returns changes with too many args', () => {
      expect(checker(20, 2, 3, 4)).toBe(20);
      expect(checker(20, 2, 30, 5)).toBe(30);
    });

    it('returns undefined values', () => {
      expect(typeof checker(1, undefined, 3, 4)).toBe('undefined');
    });
  });
});