casca.js

const dictionary = "abcdefghijklmnopqrstuvwxyz";
const create = letter => {
  [...dictionary].map((letterDict, indexLetter) => {
    if (letter.toLowerCase() == letterDict) {
      createCascate(indexLetter);
    }
  });
};

const createCascate = (indexLetter) => {
  const numberOfIterations = indexLetter + 1;
  const array = [...new Array(numberOfIterations * 2 +1)];
  for (let index = 0; index <= indexLetter; index++) {
    array.fill("_")
    array[indexLetter + index + 1] = dictionary[index];
    array[indexLetter - index + 1] = dictionary[index];
    console.log(array.join(" "));
  }
  for (let index = indexLetter-1; index >= 0; index--) {
    array.fill("_")
    array[indexLetter + index + 1] = dictionary[index];
    array[indexLetter - index + 1] = dictionary[index];
    console.log(array.join(' '));
  }
};

create("j");
create("a");
// _ _ _ _ _ _ _ _ _ A _ _ _ _ _ _ _ _ _
// _ _ _ _ _ _ _ _ B _ B _ _ _ _ _ _ _ _
// _ _ _ _ _ _ _ C _ _ _ C _ _ _ _ _ _ _
// _ _ _ _ _ _ D _ _ _ _ _ D _ _ _ _ _ _
// _ _ _ _ _ E _ _ _ _ _ _ _ E _ _ _ _ _
// _ _ _ _ F _ _ _ _ _ _ _ _ _ F _ _ _ _
// _ _ _ G _ _ _ _ _ _ _ _ _ _ _ G _ _ _
// _ _ H _ _ _ _ _ _ _ _ _ _ _ _ _ H _ _
// _ I _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ I _
// J _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ J
// _ I _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ I _
// _ _ H _ _ _ _ _ _ _ _ _ _ _ _ _ H _ _
// _ _ _ G _ _ _ _ _ _ _ _ _ _ _ G _ _ _
// _ _ _ _ F _ _ _ _ _ _ _ _ _ F _ _ _ _
// _ _ _ _ _ E _ _ _ _ _ _ _ E _ _ _ _ _
// _ _ _ _ _ _ D _ _ _ _ _ D _ _ _ _ _ _
// _ _ _ _ _ _ _ C _ _ _ C _ _ _ _ _ _ _
// _ _ _ _ _ _ _ _ B _ B _ _ _ _ _ _ _ _
// _ _ _ _ _ _ _ _ _ A _ _ _ _ _ _ _ _ _