```javascript
function goodKeys(obj, callback) {
  let array = [];
  // loop through each property obj
  for (let property in obj) {
    // if the return value of the callback equals true
    if (callback(obj[property]) === true) {
      // then push key to array
      array.push(property);
    }
  }
  return array;
}
// Uncomment these to check your work!
const sunny = { mac: 'priest', dennis: 'calculator', charlie: 'birdlaw', dee: 'bird', frank: 'warthog' };
function startsWithBird(str) { return str.slice(0, 4).toLowerCase() === 'bird'; };
console.log(goodKeys(sunny, startsWithBird)); // should log: ['charlie', 'dee']
```