Multiplication with no multiplication, using range reduce

multiplication.js

const isInt = n => parseInt(n) === n;

const toPrecisionN = precision => n =>
  Number((n).toPrecision(precision));

function multiplyInt(a, b) {
  const iterable = [...new Array(Math.abs(Math.floor(a)))];
  return iterable.reduce(
    acc => a < 0
      ? acc - b 
      : acc + b, 
      0
  );
}


function multiply(a, b, acc = undefined) {
  if (isInt(b) && !isInt(a)) {
    // invert operands for algorhythmic short-circuit
    return multiplyInt(b, a, acc);
  }

  const precision = 5;
  const toPrecision = toPrecisionN(precision);
  
  const intProduct = multiplyInt(a, b, acc);
  
  if (!isInt(a)) {
    const rest = parseInt(a) - a;
    const precisionFactor = Math.pow(10, precision);
    const restProduct = multiplyInt(rest * precisionFactor, b);
    const absRestProduct = Math.abs(restProduct / precisionFactor);
    
    return toPrecision(intProduct + absRestProduct);
  } else {
    return toPrecision(intProduct);
  }
}