AI solution for JavaScript Coding Question on BFE.dev
160. implement atob()

function myAtob(str) {
  const base64Chars =
    'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';
  let decodedStr = '';
  let bits;

  for (let i = 0; i < str.length; i++) {
    const charIndex = base64Chars.indexOf(str[i]);
    if (charIndex === -1) {
      throw new Error('Invalid Base64 input');
    }
    bits = i % 4 === 0 ? charIndex : bits * 64 + charIndex;
    if (i % 4 === 3) {
      decodedStr += String.fromCharCode((bits >> 16) & 255, (bits >> 8) & 255, bits & 255);
    }
  }

  if (str.endsWith('==')) {
    decodedStr = decodedStr.slice(0, -2);
  } else if (str.endsWith('=')) {
    decodedStr = decodedStr.slice(0, -1);
  }

  return decodedStr;
}

Explanation:

The myAtob function takes in a string that needs to be decoded. We then create a string called base64Chars that contains all the 64 characters used in Base64 encoding along with the padding character.

We then iterate over the input string, extracting each character's corresponding index in the base64Chars string. We then convert the indices into their respective binary values, storing the bits in the bits variable. If the current iteration is a multiple of 4, then we know that we have read in 3 characters (24 bits), which we can then convert to utf-8 string representation using String.fromCharCode(). We reset the bits variable so that we can start reading in the next set of characters.

After we have read in all the characters, we trim the decoded string depending on whether the original input string had 1 or 2 padding characters. Finally, we return the decoded string. If the input string contains any invalid characters, we throw an error with an appropriate message.