function filter(arr, filterCallback) {
  // First, we check if the parameters passed are right.
  if (!Array.isArray(arr) || !arr.length || typeof filterCallback !== 'function') 
  {
    return [];
  } else {
    let result = [];
    // We're making a results array every time we call this function
    // because we don't want to mutate the original array.
    for (let i = 0, len = arr.length; i < len; i++) {
      // check if the return value of the filterCallback is true or "truthy"
      if (filterCallback(arr[i], i, arr)) { 
      // push the current item in the 'result' array if the condition is true
        result.push(arr[i]);
      }
    }
    return result; // return the result array
  }
}