1:  const alpha = ['a', 'b', 'c'];

   2:  const numeric = [1, 2, 3];

   3:  numeric[Symbol.isConcatSpreadable] = false;  // numeric won't be flattened during concatenation

   4:  let alphaNumeric = alpha.concat(numeric);

   5:  console.log(alphaNumeric); // Output: ['a', 'b', 'c', [1, 2, 3]].  numeric isn't flattened.

   6:  numeric[Symbol.isConcatSpreadable] = true; //numeric array will be spread when concatinated

   7:  alphaNumeric = alpha.concat(numeric);

   8:  console.log(alphaNumeric); // ['a', 'b', 'c', 1, 2, 3];