0

function reverse(num) {
  let newwarray = [];
  let temparray = [];
  var x = num.toString();
  for (var i = x.length - 1; i >= 0; i--); {
    newwarray += x[i]
  }
  return newwarray;
}
document.write(reverse(3456))

So the title basically says it all. The result is undefined and I can't find the problem by using Console.

Nick Parsons
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Jiang Yuxin
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3 Answers3

2

There are two little mistakes you've made.

The first one is the semicolon at the end of the line here:

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

Due to that, the stuff in the brackets in the following lines, is not the loops body. It's a seperate statement. The loops body is that single semicolon ;.

Therefore what happens is that the loop runs, till the end condition is met (when i is -1), then the block behind it runs, and does:

{
 newwarray+=x[i]
}

And the -1 position of x is undefined. Now if you add an array and undefined, both the array and undefined will be casted to a string (an empty array is turned into an empty string), and you end up with "undefined".

To make it working, remove the semicolon, and .push to the array:

for (var i = x.length - 1; i >= 0; i--) {
  newwarray.push(x[i]);
}

As a sidenote, the first mistake was only possible because you used var, which declares i not only in the loop's body, but in the whole function. I strongly recommend using let to spot such errors more easily:

 for(let i = 1; i < 0; );
 {
   console.log(i); // ReferenceError: i is not defined
 }
Jonas Wilms
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2

If you have to reverse the number then you can make use of Array.prototype.reverse function by doing a trick

function reverse(num){
  let arr = num.toString().split("");
  arr = arr.reverse();
  return arr.join("")
}
document.write(reverse(3456)); //6543
Akshay Bande
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1

Try this, it reverses the number by making a new array

var number = 12345;
var stringArray = number.toString().split("");
var generatedReverse = [];

 /* Loop through array of numbers */
for(i = 0; i < stringArray.length; i++){
    var rev = stringArray[stringArray.length - 1 - i];
    generatedReverse.push(rev);
}
var reverseFinal = generatedReverse.join("");
document.write(reverseFinal);
Von Uyvico
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