文字列を取得して表示する関数repeatAlphaを作成します
各アルファベット文字をそのアルファベットのインデックスと同じ回数繰り返します。

解決

const range = (start, stop, step) =>
  Array.from(
    { length: Math.ceil((stop - start) / step) },
    (_, i) => start   i * step
  );

const upperAlpha = range("A".charCodeAt(0), "Z".charCodeAt(0)   1, 1).map((x) =>
  String.fromCharCode(x)
);

const lowerAlpha = range("a".charCodeAt(0), "z".charCodeAt(0)   1, 1).map((x) =>
  String.fromCharCode(x)
);

function getAlphaIndex(char) {
  if (char === char.toUpperCase()) {
    return upperAlpha.indexOf(char)   1;
  }

  if (char === char.toLowerCase()) {
    return lowerAlpha.indexOf(char)   1;
  }
}

function repeatAlpha(text) {
  let occurrence = [];

  Array.from(text).forEach((char) => {
    let count = getAlphaIndex(char);
    let result = Array(count).fill(char).join("");
    occurrence.push(result);
  });

  return occurrence.join("");
}

console.log(repeatAlpha("Becky"));
console.log(repeatAlpha("neNgi"));
console.log(repeatAlpha("ChInwendu"));
console.log(repeatAlpha("dindustack"));

結果

BBeeeeeccckkkkkkkkkkkyyyyyyyyyyyyyyyyyyyyyyyyy
nnnnnnnnnnnnnneeeeeNNNNNNNNNNNNNNgggggggiiiiiiiii
CCChhhhhhhhIIIIIIIIInnnnnnnnnnnnnnwwwwwwwwwwwwwwwwwwwwwwweeeeennnnnnnnnnnnnndddduuuuuuuuuuuuuuuuuuuuu
ddddiiiiiiiiinnnnnnnnnnnnnndddduuuuuuuuuuuuuuuuuuuuusssssssssssssssssssttttttttttttttttttttaccckkkkkkkkkkk