return statement in function that convert decimal to binary numbers

I’m trying to understand how to use functions in JS.

This code converting decimal numbers to binary numbers:

for (j = 13; j <= 16; j++) {
  res = ""
  number = j

  while (number > 0) {
    res = res + number % 2
    number = Math.floor(number / 2)
  }

  len = res.length

  rev=""

  for (i = 1; i <= len; i++) {
    rev = rev + res[res.length - i]
  }
  
  console.log(rev)
}

but when I’m trying to put this code into a function, the function returns only the first or the last value. What am I doing wrong?

function f(min, max) {
  for (j = min; j <= max; j++) {
    res = ""
    number = j
    
    while (number > 0) {
      res = res + number % 2
      number = Math.floor(number / 2)
    }

    len = res.length

    rev=""

    for (i = 1; i <= len; i++) {
      rev = rev + res[res.length-i]     
    }  
  }
  return rev
}

console.log(f(13,15))

Answer

You have to store the result of each iteration in array.

function f(min, max) {
  const results = []
  for (j = min; j <= max; j++) {
    res = ""
    number = j
    
    while (number > 0) {
      res = res + number % 2
      number = Math.floor(number / 2)
    }

    len = res.length

    rev=""

    for (i = 1; i <= len; i++) {
      rev = rev + res[res.length-i]     
    }
    
    results.push(rev)
  }
  return results
}

console.log(f(13,15))

Also you should declare your variables in JavaScript, because not doing that can lead to bugs https://www.geeksforgeeks.org/what-happen-when-we-directly-assign-the-variable-without-declaring-it-in-javascript/