function highlight(strings, ...values) {
  return strings.reduce((result, string, i) => {
    const value = values[i] ? `<mark>${values[i]}</mark>` : '';
    return result + string + value;
  }, '');
}

const name = 'Sarah';
const age = 25;
const template = highlight`Hello ${name}, you are ${age} years old!`;
console.log(template);

const empty = highlight`No interpolation here`;
console.log(empty);