text: var chai = require('chai');
  var assert = chai.assert;
  
  const e = require('./solution.js');
  
  describe('Add, Subtract, Multiply or Divide? (unit tests)\n', () => {
    it('[Test 1] Expected: "added"', () =>
      assert.strictEqual(e.operation(12, 12), 'added'));
    it('[Test 2] Expected: "subtracted"', () =>
      assert.strictEqual(e.operation(100, 76), 'subtracted'));
    it('[Test 3] Expected: "multiplied"', () =>
      assert.strictEqual(e.operation(6, 4), 'multiplied'));
    it('[Test 4] Expected: "divided"', () =>
      assert.strictEqual(e.operation(528, 22), 'divided'));
    it('[Test 5] Expected: null', () =>
      assert.strictEqual(e.operation(10, 12), null));
  });
  