2

I wrote a JS constructor which reverse a string variable:

function ReverseString(string) {
    this.str = string;
    var size = this.str.length;
    this.reverse = function () {
        for(size; size >= 0; --size) {
            console.log(this.str[size]);
        }
    }
}

When I invoke a reverse method on a new string object("asd") it produces the following output:

  undefined 
    d 
    s 
    a

Where this undefined came from? Could you help me eliminate this ?

Kamil Kiełczewski
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RandomUser
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    The range of characters indexes in a string goes from zero to the string length, **minus one**. – Frédéric Hamidi Sep 29 '14 at 12:43
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    Also, just in case you don't want to reinvent the wheel, `str.split("").reverse().join("")` is an useful idiom. – Frédéric Hamidi Sep 29 '14 at 12:44
  • possible duplicate of [How do you reverse a string in place in JavaScript?](http://stackoverflow.com/questions/958908/how-do-you-reverse-a-string-in-place-in-javascript) –  Sep 29 '14 at 12:46
  • Just curious, any particular reason you're inventing an entirely new type of object `ReverseString` with a single purpose in life which is to reverse the string it holds? You plan to reverse it again and again? –  Sep 29 '14 at 12:47
  • Let's see, you searched for "reverse string javascript" before posting this question and found nothing, I guess. Seems no-one ever had this problem before. Except http://stackoverflow.com/questions/1611427/reversing-a-string-in-javascript, http://stackoverflow.com/questions/4859208/recursive-string-reversal-function-in-javascript (see duplicate answer below), and off-site resources such as http://eddmann.com/posts/ten-ways-to-reverse-a-string-in-javascript/. –  Sep 29 '14 at 12:53

8 Answers8

5

The length problem is already explained. To solve it you could use:

function ReverseString(string) {
    this.str = string;
    var size = this.str.length;
    this.reverse = function () {
        while(size--) {
            console.log(this.str[size]);
        }
    }
}

Another (more simple way) to reverse a string is to split it into an array, reverse that, and join again:

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

Applied to your method, something like this snippet:

var result = document.querySelector('#result');

function ReverseString(string) {
  this.str = string;
  this.reverse = function() {
    var rev = this.str.split('').reverse();
    void(rev.map(function(v) {
     result.innerHTML += v + '<br>';
    }));
    this.reversed = rev.join('');
  };
}

var myReversed = new ReverseString('hello world');
myReversed.reverse();
<div id="result"></div>
KooiInc
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2

At the beginning, size is going to be 3 (the length). There is nothing at index 3, hence undefined. You need to initiate it at length-1.

var size = this.str.length - 1;
Scimonster
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    I think a better idea would be to either fix the condition in the loop or give the variable a different name. I don't like the idea of a variable named `size` which isn't *actually* the size (length) of the string. – Matt Burland Sep 29 '14 at 12:45
  • I agree that `size` isn't the best name, but names aren't the most important thing. The logic is. Good names help though, of course. – Scimonster Sep 29 '14 at 12:47
  • A variable called `size` that isn't actually the *size* of something sounds pretty illogical to me. – Matt Burland Sep 29 '14 at 12:52
2

Here are three ways to reverse a string in javascript

Iterate through string in reverse order and push letters to array, then join the array

function reverse1(str) {
  if(!str || str.length < 2 || typeof str !== "string") {
    return new Error("Not a valid input");
  }
  const backwards = [];
  for (let i = str.length; i > -1; i--) {
    backwards.push(str[i]);
  }
  return backwards.join('');
}

Use split, reverse, and join methods

function reverse2(str) {
  return str.split().reverse().join();
}

Use ES6 arroe function and spread operator to copy as well as convert the string to array and then reverse and join

reverse3 = str => [...str].reverse().join('');

Call the three functions

console.log(reverse1("Hello World!"));
console.log(reverse2("Hello World!"));
console.log(reverse3("Hello World!"));

Output

!dlroW olleH
Hello World!
!dlroW olleH
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    `reverse2` (as even your Output shows) doesn't work because you haven't specified arguments for splitting and joining. `function reverse2(str) { return str.split().reverse().join(); }` – OpaCitiZen Sep 24 '22 at 12:06
2

What about this easy step

"hello world".split(' ').map(word=>word.split('').reverse().join('')).join(' ')

it will reverse every word in its place

"olleh dlrow"
Omar Hasan
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    You seem to have left out the actual reversing step: `"hello world".split(' ').map(word=>word.split('').reverse().join('')).join(' ')` – OpaCitiZen Sep 24 '22 at 11:52
1

User recusrsivity, easiest way to do it

function reverse(str){
     if (str.length == 0){ 
        return '';
     }
     return reverse(str.slice(1)) + charAt(0);
}
user28470
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  • Instead of `str.substring(1, str.length)`, just `str.slice(1)`. You're also missing parens around your `if` clause. –  Sep 29 '14 at 12:48
  • I don't think the OP was looking for an alternative implementation but was rather asking why their implementation gave some unexpected results. – Matt Burland Sep 29 '14 at 12:51
  • Thanks, changed :) I'm doing delphi at the moment used to write with no parens x) – user28470 Sep 29 '14 at 12:51
  • @MattBurland Yup, just giving another solution faster to write. recusrsivity can be usefull :3 – user28470 Sep 29 '14 at 12:52
1

I would use reduceRight:

function sum(arr) {
    return arr.reduceRight(function(result, chr) {
        return result += chr;
    });
}

Now I have a handy routine I can also use to sum up sequences of numbers (in reverse order, but that doesn't matter).. For string:

function reverse_string(str) {
    return sum(str.split(''));
}
1

Try as altenrative

[...str].reverse().join``;

In your solution size at beginning should be length -1. You mix string reversing with string printing, but you can separate it as follows

let str = "Hello World!";
let r = [...str].reverse().join``;
console.log(r);
Kamil Kiełczewski
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0

In here you have get the size of the 'string' constructor parameter and you decrese the length of the given word('size' variable).

function ReverseString(string)     { 
    this.str = string; 
    var size = this.str.length;
    this.reverse = function () {
        for(size; size >= 0; --size){ //loop from index value 3 to 0 
          console.log(this.str[size]);
        } 
    }
}

But the problem is when you input word 'asd', the length of that string is 3. So, when you access the index of str[3] should be undefined. Because,

    index[0] --> a
    index[1] --> s
    index[2] --> d
    index[3] --> undfined

So, your code should change,

function ReverseString(string)   { 
    this.str = string; 
    var size = this.str.length; 
    this.reverse = function () {
        for(size; size > 0; --size) { //loop from index value 2 to 0 
            console.log(this.str[size - 1]);
        } 
    }
}

So, output should be like this,

    index[0] --> a
    index[1] --> s
    index[2] --> d
Geeganage Punsara Prathibha
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