```javascript
function groupBy(array, callback) {
  let obj = {};
  let returnArray = [];
  // loop through array and perform callback on each element
  array.forEach(function(element) {
    // create array of return values
    let returnValue = callback(element);
    returnArray.push(returnValue);
  });

  // loop through each return value
  for (let i = 0; i < returnArray.length; i++) {
    // initialize property array
    let propertyArray = [];
    // loop through each element
    for (let j = 0; j < array.length; j++) {
      // if return of callback function equals return, then push element to property array
      if (callback(array[j]) === returnArray[i]) {
        propertyArray.push(array[j]);
      }
    }
    // assign property array to appropriate key
    obj[returnArray[i]] = propertyArray;
  }
  return obj;
}
// Uncomment these to check your work!
const decimals = [1.3, 2.1, 2.4];
const floored = function(num) { return Math.floor(num); };
console.log(groupBy(decimals, floored)); // should log: { 1: [1.3], 2: [2.1, 2.4] }
```