Does condition expression of for loop changes after each iteration?

I'm trying to write a function decreasingOrder which takes a positive integer as input and return an array of its digits in decreasing order.

e.g., decreasingOrder(1234) Should give [4,3,2,1].

function decreasingOrder(n) {
  let unarr = [...`${n}`].map(i => parseInt(i)); //Unordered Array of Digits
  let oarr = []; //Ordered Array of Digits
  
  for(let j=0; j<unarr.length; j++){
  let max = Math.max.apply(Math, unarr);
  oarr.push(max);
  unarr.splice(unarr.indexOf(max), 1); //delete element from array
  }

  return oarr;
}

console.log(decreasingOrder(1234));
//Expected [4,3,2,1], Instead got [4,3]

I think, deleting element using splice method also reduces the number of iteration.
I also tried delete operator but get [4, NaN, NaN, NaN] (because Math.max([undefined])).
When I tried with specific number instead of unarr.length in condition expression for for loop, it works fine!

So when I use splice method to delete elements it reduces the unarr.length and when I tried to keep unarr.length constant using delete operator it gives NaN, what should I do? Is there any other way to write to the same function? I'm beginner in JavaScript.

Solution

The issue in your code is unarr.splice(unarr.indexOf(max), 1) inside loop.

By taking your example of console.log(decreasingOrder(1234)). In the first cycle the highest number from the array is found and is removed from the array and pushed to new array.

At the end of the first cycle the outputs will be unarr = [1, 2, 3], oarr = [4] and j=1

Likewise after second loop unarr = [1, 2], oarr = [4, 3] and j=2. Now the loop condition j < unarr.length is not satisfied hence the loop breaks. So the output will be [4, 3].

Instead you can use the below utility for your requirement.

function decreasingOrder(n) {
  let unarr = [...`${n}`].map(i => parseInt(i)) //Unordered Array of Digits

  return unarr.sort((a,b) => b-a)
}

console.log(decreasingOrder(1234))

Hope this helps.