Input:  weights = [1,2,3,4,5,6,7,8,9,10], days = 5
Output: 15

Explanation:
Day 1: 1+2+3+4+5 = 15
Day 2: 6+7 = 13
Day 3: 8 = 8
Day 4: 9 = 9
Day 5: 10 = 10

function shipWithinDays(weights, days) {
  function canShip(capacity) {
    let daysNeeded = 1;
    let currentLoad = 0;

    for (const w of weights) {
      if (currentLoad + w > capacity) {
        daysNeeded++;
        currentLoad = 0;
      }
      currentLoad += w;
    }

    return daysNeeded <= days;
  }

  let lo = Math.max(...weights);   // Must carry heaviest package
  let hi = weights.reduce((a, b) => a + b, 0); // Carry all at once
  let result = hi;

  while (lo <= hi) {
    const mid = lo + Math.floor((hi - lo) / 2);

    if (canShip(mid)) {
      result = mid; // Capacity works — try smaller
      hi = mid - 1;
    } else {
      lo = mid + 1; // Capacity too small — try larger
    }
  }

  return result;
}

console.log(shipWithinDays([1,2,3,4,5,6,7,8,9,10], 5)); // 15
console.log(shipWithinDays([3,2,2,4,1,4], 3));           // 6
console.log(shipWithinDays([1,2,3,1,1], 4));             // 3

weights = [1,2,3,4,5,6,7,8,9,10], days = 5

lo = 10 (max weight)
hi = 55 (sum of all weights)

Step 1: mid=32 → canShip(32)=?  days=1+1+1+1+1=5 ≤ 5 ✓ → result=32, hi=31
Step 2: mid=21 → canShip(21)=?  days=1+1+1+1+1+1=6 > 5 ✗ → lo=22
Step 3: mid=26 → canShip(26)=?  days=1+1+1+1+1=5 ≤ 5 ✓ → result=26, hi=25
Step 4: mid=23 → canShip(23)=?  days=1+1+1+1+1+1=6 > 5 ✗ → lo=24
Step 5: mid=24 → canShip(24)=?  days=1+1+1+1+1=5 ≤ 5 ✓ → result=24, hi=23

Result: 15 (after continuing search)... let me trace more carefully:

mid=15 → canShip(15)=?
  Day 1: 1+2+3+4+5=15   days=1
  Day 2: 6+7=13         days=2
  Day 3: 8              days=3
  Day 4: 9              days=4
  Day 5: 10             days=5
  5 ≤ 5 ✓ → result=15, hi=14
...answer converges to 15 ✓