Reverse a string in javascript

Question

I borrowed a piece of code for reversing a string in JavaScript. However it can't be done.

                <!DOCTYPE HTML>
            <html>
            <head>
            <meta charset="utf-8">
            <title>P4--Largest palindrome product</title>
            </head>
            <body>
            <!--A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.

            Find the largest palindrome made from the product of two 3-digit numbers.
             --> 
            <script type="text/javascript">
            var newdiv = document.createElement('div');
            newdiv.innerHTML=Palindromic();
                alert(Palindromic());
            function Palindromic(){
                var x;
                var a;
                var b;
                for(i=999*999;i>=100*100;i--)
                {
                    x=i.toString();
                    if(x != x.reverse())
                       continue;
                    else
                    {
                        for(j=100;j<999;j++)
                        {
                            if(i%j == 0)
                            {
                                var y =i/j;
                                if(y>=100 && y<=999)
                                {
                                    a=i;b=y;
                                    break;
                                }
                            }
                        }
                    }
                }
                return x;
            }
            String.prototype.reverse = function() {
              return this.split('').reverse().join('');
            }
            </script>
            </body>
            </html>

The error is "has no method 'reverse'".

Probably because you're calling the function before you're declaring it.. — Mike Christensen
– Mike Christensen, Commented Jan 28, 2014 at 16:54
Note: It is a bad practice to modify native definitions. — Florent
– Florent, Commented Jan 28, 2014 at 16:57
Note: It is a bad practice to go around saying "X is a bad practice" just cause you read it somewhere. — cHao
– cHao, Commented Jan 28, 2014 at 17:03

Peter Olson · Accepted Answer · 2014-01-28 16:54:38Z

3

You need to define the String.prototype.reverse function before you use it.

Put this code

String.prototype.reverse = function() {
  return this.split('').reverse().join('');
}

at the top instead of at the bottom.

answered Jan 28, 2014 at 16:54

Peter Olson

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2 Comments

user1108948 Over a year ago

Yes, you are right. One more question if I could get the answer, it crashes the Firefox because timeout. Any improvement for the algorithm?

Peter Olson Over a year ago

@Love It looks like you are trying to solve Problem 4 from Project Euler. I don't really want to give away a solution algorithm here on Stack Overflow, but if you want solutions, searching for "JavaScript Project Euler problem 4" yields plenty of results.

Durja · Accepted Answer · 2017-12-04 23:02:55Z

1

ES6: try this function

function reverse(str) {
           // 1 //    // 2 //   // 3 //
    return [...str].reverse().reduce((rev,el) => rev + el,'');
}

spreads the string into an array of chars
reverses the array
reduce method on array to get the previous value and accumulate.

edited Dec 4, 2017 at 23:02

answered Dec 4, 2017 at 21:49

Durja

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2 Comments

Stephen Ostermiller Over a year ago

Can you explain a bit more about what this does? You are using quite a few obscure syntaxes for just one line. It isn't obvious to me how it helps with emojis or weird characters.

Durja Over a year ago

Thanks Stephen. Added some helpers. Please let me know if you've got it and if you did - you can up vote. Regarding my warning on split(), it's from my personal experience where I had issues. I don't have the code handy but the above code uses ES6 syntax to reverse a string.

Nechar Joshi · Accepted Answer · 2022-04-11 06:46:27Z

0

Here is the easiest approach:

// reverse a string
function reverseString(str) {
  return str.split('').reverse().join('');
}

const reversedString = reverseString('hello world');
console.log(reversedString); // this would be logged as 'dlrow olleh'

answered Apr 11, 2022 at 6:46

Nechar Joshi

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