Function to transform string into base64

base64.js

/**
 *
 * @param {string} str
 */
function base64(str) {
  const base64Characters = [
    "A",
    "B",
    "C",
    "D",
    "E",
    "F",
    "G",
    "H",
    "I",
    "J",
    "K",
    "L",
    "M",
    "N",
    "O",
    "P",
    "Q",
    "R",
    "S",
    "T",
    "U",
    "V",
    "W",
    "X",
    "Y",
    "Z",
    "a",
    "b",
    "c",
    "d",
    "e",
    "f",
    "g",
    "h",
    "i",
    "j",
    "k",
    "l",
    "m",
    "n",
    "o",
    "p",
    "q",
    "r",
    "s",
    "t",
    "u",
    "v",
    "w",
    "x",
    "y",
    "z",
    "0",
    "1",
    "2",
    "3",
    "4",
    "5",
    "6",
    "7",
    "8",
    "9",
    "+",
    "/",
  ];

  const binariesGroups24bits = str.split("").reduce((groups, c) => {
    const ascii = c.charCodeAt(0);
    const binary = ascii.toString(2).padStart(8, "0");

    if (groups.length === 0 || groups[groups.length - 1].length >= 24)
      groups.push(binary);
    else groups[groups.length - 1] = `${groups[groups.length - 1]}${binary}`;

    return groups;
  }, []);

  return binariesGroups24bits
    .map((group) => {
      const firstChar = group.substr(0, 6);
      let secondChar;
      let thirdChar;
      let forthChar;

      switch (group.length) {
        case 8:
          secondChar = group.substr(6).padEnd(6, "0");
          break;
        case 16:
          secondChar = group.substr(6, 6);
          thirdChar = group.substr(12).padEnd(6, "0");
          break;
        case 24:
          secondChar = group.substr(6, 6);
          thirdChar = group.substr(12, 6);
          forthChar = group.substr(18).padEnd(6, "0");
          break;
      }

      return [firstChar, secondChar, thirdChar, forthChar]
        .map((char) =>
          typeof char === "undefined"
            ? "="
            : base64Characters[parseInt(char, 2)]
        )
        .join("");
    })
    .join("");
}

const input = "light work.";
const output = base64(input);

console.log(
  `input: ${input}\n\noutput: ${output}\n\ndoes it work? `,
  output === "bGlnaHQgd29yay4="
);