Javascript - Undefined array value after string .split

Question

I wanted to check if a string is palindrome, but I have a problem in reversing a string. I know there's a lot of Questions about palindrome in Javascript, but i want to find it on my way and I still can't find the solution on those Questions.

Code

function reverse(string){
  var str = string;
  var split = str.split("");
  var newStr= "";

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

  console.log(newStr); 
}

var str= 'blue';

reverse(str);

Results

"undefinedeulb"

Question

As we can see, the results was an undefined value at the start of the string. I tested this code with PHP script, and It works fine. How could this be a problem with Javascript? and How to get rid of it?

You are trying to access split[split.length] which is undefined. You are off by one — Andrew Li
– Andrew Li, Commented Oct 19, 2016 at 22:18
FYI, the reason why it seems to work in PHP is that the undefined index seems to be coerced to an empty string somehow. It's still wrong though. — Felix Kling
– Felix Kling, Commented Oct 19, 2016 at 22:45

Will P. · Accepted Answer · 2016-10-19 23:56:36Z

1

The index of an array starts with 0, for example in

var arr = ["a", "b", "c", "d", "e"];

the last one is arr[4] // "e"

but arr.length is 5, so arr[5] is undefined.

So arr[arr.length] is always undefined!

In your example the first iteration, i equals split.length is undefined, therefore try

for(var i = split.length - 1; i >= 0; i--).

edited Oct 19, 2016 at 23:56

user3589620

answered Oct 19, 2016 at 22:18

Will P.

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2 Comments

Felix Kling Over a year ago

Alternatively: for(var i=split.length; i--;) {}

Will P. Over a year ago

That's true, and I also agree with adeneo's comment that there is a much simpler way to implement this functionality, but since Billy wanted to figure it out themselves I figured I'd leave it as close to their implementation as possible

JeremyS · Accepted Answer · 2016-10-19 22:22:16Z

0

split.length is equal to 4, but because arrays are zero-indexed you need to start at 3 and move down to zero.

function reverse(string){
  var str = string;
  var split = str.split("");
  var newStr= "";

  for(var i = split.length -1; i >= 0; i--){
     newStr += split[i]; console.log(i);
  }

  console.log(newStr); 
}

var str= 'blue';

reverse(str);

answered Oct 19, 2016 at 22:22

JeremyS

4372 silver badges7 bronze badges

1 Comment

Billy Over a year ago

Exactly! Thanks Jeremy S

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