Returns an equivalent fraction to a number larger than 0 (limited by a rather limited call stack size cap)

fractions.annotated.js

f = (                          // Given
  n,                           // A number greater than 0 (limited by the call stack size cap)
  a,                           // An optional numerator
  b                            // And an optional denominator
) =>
  [                            // Have a collection of functions that
    _ => f(n, a + 1, b),       // - Adds one to the numerator if the fraction is smaller than the number
    _ => `${a}/${b}`,          // - Returns a string of the resulting fraction if it's equal to the number
    _ => f(n, a, b + 1),       // - Adds one to the denominator if the fraction is larger than the number
  ][                           // And return the value of one based on
    Math.sign(                 // The sign of the operation resulting from having
      (a = a || 1)             //   The numerator, or 1 if none
      /                        //  Divided by
      (b = b || 1)             //   The denominator, or 1 if none
      - n                      //  Minus the number we are looking for
    ) + 1                      // Aligned to the index of the needed function
  ](console.log(`${a}/${b}`))  // Also, print every step

fractions.js

f=(n,a,b)=>[_=>f(n,a+1,b),_=>`${a}/${b}`,_=>f(n,a,b+1),][Math.sign((a=a||1)/(b=b||1)-n)+1]()