Anyone can explain why the return is [-1,-1] here?

function pair_with_target_sum(arr, targetSum) { const nums = {}; // to store numbers and their indices for (let i = 0; i < arr.length; i++) { const num = arr[i]; if (targetSum - num in nums) { return [nums[targetSum - num], i]; } nums[arr[i]] = i; } return [-1, -1]; }