When reversing string, I'm getting a NaN at the end. Javascript

Question

I'm trying to reverse a string, and it does work, but when I store the userInput[i] into the result variable, I'm getting NaN at the end of the string.

//variables
var userInput, result;

userInput = prompt("Enter a string that you want reversed: ");

for (var i = userInput.length; i >= 0; i--) {
    result += userInput[i];
}

console.log("Your string: \n" + result);

`undefined + undefined` is `NaN` – Oriol Jan 30 '17 at 13:00 — Oriol, Jan 30 '17 at 13:00

score 3 · Accepted Answer · edited May 23 '17 at 10:29

3

You start the loop from userInput.length which is a wrong index because userInput[userInput.length] is undefined. What you need to do is start from userInput.length - 1 like this (and don't forget to initialize result to empty string before starting to accumulate the result):

result = "";
for (var i = userInput.length - 1; i >= 0; i--) {
  result += userInput[i];
}

NOTE: When going up an array, we don't actually reach the point when we access the userInput.length index (i.e. userInput[userInput.length]) because usually the condition for the loop is i < userInput.length which fail as soon as i is equal to userInput.length. When going downwards an array, one should keep track of what are the indexes allowed.

NOTE 2: It is safer to use string.charAt(index) rather than string[index]. Learn why here.

edited May 23 '17 at 10:29

Community

1
1

answered Jan 30 '17 at 12:45

Kaushik solanki

438
3
15

this is the only correct answer so far, but it would be better you if explain what have you changed and why – georg Jan 30 '17 at 12:48
1) declare result variable as default string 2) in for loop "userInput.length - 1" Coz. array first index is 0. – Kaushik solanki Jan 30 '17 at 12:49
Why do you declare `var result` twice? – Pavlo Jan 30 '17 at 12:52
coz, Initialize it to be an empty string so JavaScript knows it should handle it as a string. – Kaushik solanki Jan 30 '17 at 12:57

score 2 · Answer 2 · answered Jan 30 '17 at 12:48

2

You are better off, by reversing a string in JavaScript using something like below:

'YOURSTRING'.split('').reverse().join('');

e-g

'abcdef'.split('').reverse().join('') will give you fedcba

answered Jan 30 '17 at 12:48

defau1t

10,593
2
35
47

1

Best way to do this (Y). – Shubham Jan 30 '17 at 12:49

score 1 · Answer 3 · edited Jan 30 '17 at 12:48

1

try this

    var userInput, result = ""; // initialize this first
    
    userInput = prompt("Enter a string that you want reversed: ");
    
    for (var i = userInput.length - 1; i >= 0; i--) {
      result += userInput[i];
    }
    console.log("Your string: \n" + result);

edited Jan 30 '17 at 12:48

Sebastian Mense

73
1
7

answered Jan 30 '17 at 12:40

Ameya Deshpande

3,580
4
30
46

Great! Thank you! – John Carlson Jan 30 '17 at 12:43
@Ameya initialise the result variable `var result = ""` , unless it'll append `undefined` first. – Shubham Jan 30 '17 at 12:47
@Pavlo it is working as expected – Ameya Deshpande Jan 30 '17 at 12:50

Sebastian Mense · Answer 4 · 2017-01-30T12:53:05.583

this problem consists of 2 smaller problems.

First problem:

"result" is not initialized so it contains "undefined" which messes with the string later on. Initialize it to be an empty string so JavaScript knows it should handle it as a string since the "+"-operator is overloaded to handle addition and string concatenation.

Second problem

The index of the userInput string reaches from 0 - (userInput.length - 1), so change the loop accordingly.

var userInput, result = ""; // initialize this first

userInput = prompt("Enter a string that you want reversed: ");

// initialize i with (userInput.length - 1) since the index ends there
for (var i = userInput.length - 1; i >= 0; i--) {
  result += userInput[i];
}
console.log("Your string: \n" + result);

Nice explanation. A code sample will make it even better. – Pavlo Jan 30 '17 at 12:50 — Pavlo, Jan 30 '17 at 12:50

Clyde Lobo · Answer 5 · 2017-01-30T14:52:13.360

1

The length property of String returns the letters' count whereas the index of an Array starts from 0. So, your issue could be resolved by reducing the string length by 1 i.e. userInput.length - 1.

var userInput, result="";

userInput = prompt("Enter a string that you want reversed: ");

for (var i = userInput.length - 1; i >= 0; i--) {
  result += userInput[i];
}
console.log("Your string: \n" + result);

edited Jan 30 '17 at 14:52

answered Jan 30 '17 at 12:42

Clyde Lobo

9,126
7
34
61

score 0 · Answer 6 · answered Jan 30 '17 at 12:52

Get you string.Split it into array.reverse your array and join.Finally you will get the reversed string.

 var userInput, result = ""; // initialize this first
    
    userInput = prompt("Enter a string that you want reversed: ");
    
   result = userInput.split("").reverse().join("");
    console.log("Reversed string: \n" + result);

When reversing string, I'm getting a NaN at the end. Javascript

6 Answers6