fraction.js

function float_to_fraction (x, error) {
  if (error === undefined) {
    error = 0.000001;
  }

  var n = parseInt(Math.floor(x));
  x = x-n;
  if(x < error) {
    return [n,1];
  } else if(1-error < x) {
    return [n+1,1];
  }
  
  // The lower fraction is 0/1
  var lower_n = 0
  var lower_d = 1
  // The upper fraction is 1/1
  var upper_n = 1
  var upper_d = 1

  while(true) {
    // The middle fraction is (lower_n + upper_n) / (lower_d + upper_d)
    var middle_n = lower_n + upper_n
    var middle_d = lower_d + upper_d
    // If x + error < middle
    if(middle_d * (x + error) < middle_n) {
      // middle is our new upper
      upper_n = middle_n
      upper_d = middle_d
    } else if(middle_n < (x - error) * middle_d) {
      // Else If middle < x - error
      // middle is our new lower
      lower_n = middle_n
      lower_d = middle_d
    } else {
    // Else middle is our best fraction
      return [n * middle_d + middle_n, middle_d];
    }
  }
}

if(!process.argv[2] && !process.argv[3]) {
  console.log("Usage: node fraction.js NOMINATOR DENOMINATOR");
  process.exit();
}

var numerator = parseFloat(process.argv[2])
var denominator = parseFloat(process.argv[3])

console.log(float_to_fraction(numerator/denominator, 0.1));