===== ISTEM =====
Solve the following programming task.

TASK: Group Array Elements (groupBy)

Write a function 'groupBy(array, fnOrKey)' that takes an array of objects and either a property name or a selector function, and returns an object grouping the elements by that value.

STARTER CODE (javascript):
function groupBy(array, fnOrKey) {
  // Write your code here
}

module.exports = { groupBy };


RULES:
- Keep the function name and signature EXACTLY as given.
- Return working code only. No explanations.
- Put the code in a single ``` block.
- Try to solve it first. If you genuinely cannot, write only this single
  line instead of producing faulty code: CANNOT_SOLVE

===== HAM YANIT =====
```javascript
function groupBy(array, fnOrKey) {
  const keyFn =
    typeof fnOrKey === 'function'
      ? fnOrKey
      : typeof fnOrKey === 'string'
      ? (item) => item[fnOrKey]
      : () => undefined;

  return array.reduce((acc, item) => {
    const key = keyFn(item);
    const keyStr = String(key);
    if (!acc[keyStr]) {
      acc[keyStr] = [];
    }
    acc[keyStr].push(item);
    return acc;
  }, {});
}

module.exports = { groupBy };
```