let input = 'The quick brown fox jumps over the lazy dog.';

const vowels = ['a','e','i','o','u'];

let resultArray = [];

input.split('').forEach( (inElement) => {
  vowels.forEach( (element) => {
    if (inElement === element ) {
            switch(element) {
        case 'e': case 'u':
          resultArray.push(element, element);
          break;
        default:
          resultArray.push(element);
          break;
      }
    }
  });
});

console.log(resultArray.join('').toUpperCase());