var from_b58 = function( S, //Base58 encoded string input A //Base58 characters (ie "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz") ) { var d = [], //the array for storing the stream of decoded bytes b = [], //the result byte array that will be returned i, //the iterator variable for the base58 string j, //the iterator variable for the byte array (d) c, //the carry amount variable that is used to overflow from the current byte to the next byte n; //a temporary placeholder variable for the current byte for(i in S) { //loop through each base58 character in the input string j = 0, //reset the byte iterator c = A.indexOf( S[i] ); //set the initial carry amount equal to the current base58 digit if(c < 0) //see if the base58 digit lookup is invalid (-1) return undefined; //if invalid base58 digit, bail out and return undefined c || b.length ^ i ? i : b.push(0); //prepend the result array with a zero if the base58 digit is zero and non-zero characters haven't been seen yet (to ensure correct decode length) while(j in d || c) { //start looping through the bytes until there are no more bytes and no carry amount n = d[j]; //set the placeholder for the current byte n = n ? n * 58 + c : c; //shift the current byte 58 units and add the carry amount (or just add the carry amount if this is a new byte) c = n >> 8; //find the new carry amount (1-byte shift of current byte value) d[j] = n % 256; //reset the current byte to the remainder (the carry amount will pass on the overflow) j++ //iterate to the next byte } } while(j--) //since the byte array is backwards, loop through it in reverse order b.push( d[j] ); //append each byte to the result return new Uint8Array(b) //return the final byte array in Uint8Array format } // example var mapping = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz"; console.log(from_b58("9vpsmXhZYMpvhCKiVoX5U8b1iKpfwJaFpPEEXF7hRm9N", mapping));