Spares to keep for a fleet

easymachinesJavaScript

To scale a fleet safely, buying 10% extra units as spares keeps the line alive while one fails. Return the total units to purchase.

Spares are ten percent, rounded up — a single unit has to be available even for a tiny fleet.
The answer is units plus the rounded-up spare count.

sparesForScale(units: int) → int

Where you start

function sparesForScale(units) {
  
}

Ceiling of ten percent in integers is (units + 9) / 10.

Reference solution in JavaScript

function sparesForScale(units) {
  return units + Math.floor((units + 9) / 10);
}