Let's say we are making a rpg hack and slash game, there are 5 item drops. Drop chances differs. This function calculates which item will drop based on their probabilities. I didn't write it, chatGPT did.

roll for item drop

function rollForDrop() {
  // List of items, with their drop chances as weights
  const items = {
    "Sword": 0.2,
    "Shield": 0.1,
    "Potion": 0.3,
    "Gold": 0.25,
    "Gem": 0.15
  };

  // Calculate total weight of all items
  const totalWeight = Object.values(items).reduce((acc, val) => acc + val, 0);

  // Roll the die and determine which item dropped
  const roll = Math.random() * totalWeight;
  let weightSum = 0;

  for (const [item, weight] of Object.entries(items)) {
    weightSum += weight;
    if (roll <= weightSum) {
      return item;
    }
  }
}