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/

You need a variable for gathering all intermediate results.

You could take declarations for all variables.

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

console.log(f(13, 15));

You should divide your code into to functions

one that turn a number in binary string (I also included the existing js version)

and one that create a list of element between min and max and iterate onto them

const toBin = n => n.toString(2)

const customToBin = n => {
  const res = []
  let number = n
  while (number > 0) {
    res.push(number % 2)
    number = Math.floor(number / 2)
  }
  return res.reverse().join('')
}

const createInterval = (min, max) => Array(max - min + 1).fill(min).map((_, i) => min + i)

const f = (min, max) => createInterval(min, max).map(customToBin)


console.log(f(13, 16))