Convert Decimal to Fraction
You can remove the toString() bit from here:
  let [int, dec] = inputNumber
    .toString()
    .split(&quot;.&quot;)
    .map((el) =&gt; parseInt(el));

toFixed returns a string. So, there is no need to call toString again.

The calculation part:
If the decimal part is 12, your output is 3/25. How do you get this? You get the greatest common divisor between 12 and 100 and divide both of them with it. GCD of 12 and 100 is 4. So, (12/4) / (100/4)
There's already a simple recursive implementation of euclidean algorithm for that:
function gcd(a, b)
    if b = 0
        return a
    else
        return gcd(b, a mod b)

You can get the precision required as an option parameter which defaults to a predefined MAX_PRECISION. Get the GCD between the dec and 10 precision (eg: 100 for precision = 2)


const MAX_PRECISION = 5

function toFraction(inputNumber, precision = MAX_PRECISION) {
  let [int, dec] = inputNumber
                      .toFixed(precision)
                      .split(".")
                      .map(n =&gt; +n)
                      
  const powerOf10 = 10 ** precision,
        gcd = getGCD(dec, powerOf10),
        fraction = `${dec/gcd}/${powerOf10/gcd}`;
  
  return int ? `${int} ${fraction}` : fraction
};

function getGCD(a, b) {
  if (!b) return a;

  return getGCD(b, a % b);
};

console.log( toFraction(1.14)   );
console.log( toFraction(5.71)   );
console.log( toFraction(3.34)   );
console.log( toFraction(5.1044) );
console.log( toFraction(0.67)   );
console.log( toFraction(0.84)   );