AI solution for JavaScript Coding Question on BFE.dev
141. implement btoa()

One possible solution for implementing btoa() is as follows:

function myBtoa(str) {
  // Define the Base64 encoding table consisting of 64 characters.
  const base64Table =
    'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';
  
  // Initialize the result string.
  let result = '';

  // Convert each character of the input string to binary representation,
  // and group them into 3-byte chunks.
  for (let i = 0; i < str.length; i += 3) {
    let chunk = str.charCodeAt(i) << 16;  // First byte of chunk.
    if (i + 1 < str.length)
      chunk |= str.charCodeAt(i + 1) << 8;  // Second byte of chunk.
    if (i + 2 < str.length)
      chunk |= str.charCodeAt(i + 2);  // Third byte of chunk.

    // Extract the 6-bit value of each Base64 digit from the chunk,
    // and append them to the result string.
    for (let j = 0; j < 4; j++) {
      if (i + j * 3 <= str.length)
        result += base64Table.charAt((chunk >> 18 - j * 6) & 0x3F);
      else
        result += '=';  // Padding for incomplete final chunk.
    }
  }
  
  return result;
}

Explanation:

1. The base64Table variable contains the 64 valid Base64 encoding characters, in the order they appear in the encoding sequence.

2. The function loops over the input string, processing it in 3-byte chunks. If the string length is not a multiple of 3, padding characters ('=') are added to the last chunk so that it has exactly 3 bytes.

3. For each chunk, the function converts it to a 24-bit binary number by concatenating the 8-bit binary representation of each of its 3 bytes.

4. The function then extracts the 6-bit value of each Base64 digit from the chunk, starting from the most significant (leftmost) digit, and appends them to the result string.

5. If the input string had to be padded with one or two extra bytes to form a complete chunk, the corresponding Base64 digits of the chunk are replaced with padding characters ('='), as per the Base64 specification.

6. Finally, the function returns the completed Base64-encoded string.

Example:

myBtoa('BFE.dev')  // Returns: "QkZFLmRldg=="

In this case, the input string has a length of 7 characters, which is not a multiple of 3. Therefore, two padding characters ('=') are added at the end of the encoded string to form a complete chunk of 3 bytes.