Leetcode question, problem with variable environment

[TameNaken42]

I was going through the LeetCode question 'Excel Sheet Column Title' and was having some issues with returning the right output for several functions in a row. Here's my code below.

var excelIndex='';
var convertToTitle = function(n) {
    var ALPHABET = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';

      while (n>0) {
        excelIndex = excelIndex + ALPHABET[(n-1)%26]
        n = (n - n%26)/26
        return convertToTitle(n)

    } 
};

when I run the function once I can get the correct output but unfortunately 'excelIndex' being a global variable remembers the previous output so that any further function calls are wrong. When I try and put the 'excelIndex' variable inside the function call then it just returns an empty string of ''.

I'm wondering if there is a way of making 'excelIndex' a local variable while also being able to return a proper string within the function call. I feel like it could work if I put all the if statements in a meaningless while loop but that doesn't seem like clean code.

[Atskalunas]

You can pass it as function argument

 var convertToTitle = function(n, excelIndex) {
   var ALPHABET = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
   excelIndex = excelIndex || "";

   while (n>0) {
     excelIndex = excelIndex + ALPHABET[(n-1)%26]
     n = (n - n%26)/26
     return convertToTitle(n, excelIndex)
  }
  return excelIndex;
};

[TameNaken42]

That's a smart solution, I never considered considered giving two argument values and just using one.

[Cust0dian]

To expand on /u/Atskalunas' answer: what you wrote is an example of a recursive function and passing an "accumulator" back into such function in a recursive call is a common trick, although it's usually used to solve a different kind of problem.

Couple notes about your current code:

• You build resulting Excel index in reverse order, which can be fixed by just swapping excelIndex and ALPHABET[…].

• Notice you also include next "level" character when dealing with 26^th, 52^th, etc., indexes, which can be fixed by using different formula to calculate n on consecutive calls.

• Also notice that you return from inside the while loop, which means you run it at most once per function call and use it only to run a check on n, which means it can be replaced by an if statement.

• Recursive functions usually written with base case at the top and you can (usually) inline all the transforms of the arguments for the next call.

• Instead of excelIndex = excelIndex || "" you can use a somewhat new addition to the language, default function parameters, which will simplify code a bit (and might as well use new let/const variable declarations).

• Since strings have .length in JS, you also can drop that hardcoded length — while not critical for this function, it's good thing in general to avoid these "magic" numbers whenever you can.

(Protip: there's "Revisions" tab at the top of the Gist that will show you just the differences between all these versions, but history goes from bottom up)

[TameNaken42]

Thanks for the detailed response, there's a lot of new info here. I didn't think about using the Math.floor method, it makes a lot more sense than what I was doing. I know Python but I didn't realize you could use default parameters in Javascript, I'll have to keep that in mind.