Implementing gotcha system.

gotcha.js

const itemArray = [
	['banana-1000', 0.65],
	['choco-960', 0.65],
	['back-0', 13.7 / 16],
	['back-1', 13.7 / 16],
	['back-2', 13.7 / 16],
	['char-0', 13.7 / 16],
	['char-1', 13.7 / 16],
	['char-2', 13.7 / 16],
	['char-3', 13.7 / 16],
	['char-4', 13.7 / 16],
	['char-5', 13.7 / 16],
	['char-6', 13.7 / 16],
	['char-7', 13.7 / 16],
	['char-8', 13.7 / 16],
	['char-9', 13.7 / 16],
	['char-10', 13.7 / 16],
	['char-11', 13.7 / 16],
	['char-12', 13.7 / 16],
	['jam-500', 2],
	['jam-300', 6],
	['jam-100', 22],
	['jam-50', 25],
	['', 30]
];
const itemProbArray = itemArray.map(value => value[1]);

itemProbArray.reduce((acc, cur, i, src) => (src[i] = acc + cur));

// ...

const mt = require('mersenne-twister');

// Sample item here;
const pivot =
  new mt().random() *
  itemProbArray[itemProbArray.length - 1];

let item = '';

for (let index = 0; index < itemProbArray.length; ++index)
  if (pivot <= itemProbArray[index]) {
    item = itemArray[index][0];
    break;
  }

print(item);