JavaScript: Get the successor of a string

JavaScript String: Exercise-48 with Solution

Write a JavaScript function to get the successor of a string.

Note: The successor is calculated by incrementing characters starting from the rightmost alphanumeric (or the rightmost character if there are no alphanumerics) in the string. Incrementing a digit always results in another digit, and incrementing a letter results in another letter of the same case. If the increment generates a carry, the character to the left of it is incremented. This process repeats until there is no carry, adding an additional character if necessary.

string.successor("abcd") == "abce"
string.successor("THX1138") == "THX1139"
string.successor("< >") == "< >"
string.successor("1999zzz") == "2000aaa"
string.successor("ZZZ9999") == "AAAA0000"

Test Data:

Sample Solution:-

HTML Code:

<!DOCTYPE html>
  <meta charset="utf-8">
  <title>JavaScript function to get the successor of a string</title>


JavaScript Code:

function successor(str) {
        var alphabet = 'abcdefghijklmnopqrstuvwxyz',
            length = alphabet.length,
            result = str,
            i = str.length;

        while(i >= 0) {
            var last = str.charAt(--i),
                next = '',
                carry = false;

            if (isNaN(last)) {
                index = alphabet.indexOf(last.toLowerCase());

                if (index === -1) {
                    next = last;
                    carry = true;
                else {
                    var isUpperCase = last === last.toUpperCase();
                    next = alphabet.charAt((index + 1) % length);
                    if (isUpperCase) {
                        next = next.toUpperCase();

                    carry = index + 1 >= length;
                    if (carry && i === 0) {
                        var added = isUpperCase ? 'A' : 'a';
                        result = added + next + result.slice(1);
            else {
                next = +last + 1;
                if(next > 9) {
                    next = 0;
                    carry = true;

                if (carry && i === 0) {
                    result = '1' + next + result.slice(1);

            result = result.slice(0, i) + next + result.slice(i + 1);
            if (!carry) {
        return result;


Sample Output:



Flowchart: JavaScript: Get the successor of a string

Live Demo:

See the Pen JavaScript Get the successor of a string-string-ex-48 by w3resource (@w3resource) on CodePen.

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JavaScript: Tips of the Day

var functionName = function() {} vs function functionName() {}

The difference is that functionOne is a function expression and so only defined when that line is reached, whereas functionTwo is a function declaration and is defined as soon as its surrounding function or script is executed (due to hoisting).

For example, a function expression:

// TypeError: functionOne is not a function

var functionOne = function() {

Historically, function declarations defined within blocks were handled inconsistently between browsers. Strict mode (introduced in ES5) resolved this by scoping function declarations to their enclosing block.

'use strict';    
{ // note this block!
  function functionThree() {
functionThree(); // ReferenceError

Ref: https://bit.ly/2zMAj1O