Base64 encode explained

String.prototype.toBase64 = function () { const base64chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdef' + 'ghijklmnopqrstuvwxyz0123456789+/'; let plaintext = this, result = '', padding = ''; // step 1. ensure that the string is a multiple of three chars if (plaintext.length % 3 > 0) { for(let count = 0; count < 3; count++) { padding += '='; plaintext += '\0'; } } // step 2. iterate over the input string, three chars at a time for (let i = 0; i < plaintext.length; i += 3) { // step 3. take three 8-bit (ASCII) chars, and store // them as one single 24-bit number // // 00000000 8-bit (1 byte) // 00000000 00000000 00000000 24-bit (3 bytes) // // we'll bitshift the first number two bytes (16 bits) // we'll bitshift the second number one byte (8 bits) // and we'll pop the other number into the third byte // // 00000000 <------- -------- first char << 16 // 00000000 <------- second char << 8 // 00000000 third char (no shift) const a = plaintext.charCodeAt(i) << 16; const b = plaintext.charCodeAt(i + 1) << 8; const c = plaintext.charCodeAt(i + 2); const n = a + b + c; // step 4. separate this 24-bit number into four 6-bit numbers // // we'll do this by shifting to the right (with zero pad >>>) // and then doing a logical AND (&) to strip everything after // the first (right most) six bits // // e.g. 00001111 00000101 00001010 // to |----||- ---||--- -||----| // d e f g // // >>> 18 00000000 00000000 00000011 // & 63 00000000 00000000 00111111 // d = 00000000 00000000 00000011 // // >>> 12 00000000 00000000 11110000 // & 63 00000000 00000000 00111111 // e = 00000000 00000000 00110000 // // >>> 6 00000000 00111100 00010100 // & 63 00000000 00000000 00111111 // f = 00000000 00000000 00010100 // // noop 00001111 00000101 00001010 // & 63 00000000 00000000 00111111 // g = 00000000 00000000 00001010 const d = (n >>> 18) & 63; const e = (n >>> 12) & 63; const f = (n >>> 6) & 63; const g = n & 63; // step 5. use the four 6-bit numbers as indices to pluck // chars from our char array above, and add to the // result string, before continuing with the loop // // this works because a 6-bit number is 0-63, and we have // 64 characters above. Therefore, we can pluck out chars result += base64chars[d]; result += base64chars[e]; result += base64chars[f]; result += base64chars[g]; } // step 6. finally, we'll remove the zero pad we added above // and add the actual padding, then return this string return result.substring(0, result.length - padding.length) + padding; };