Calculate booklet page printing order

booklet.js

// Pass in the number of pages in your document, e.g. 8

// The output is an array describing how to layout the pages for a booklet
// e.g. [8, 1, 2, 7, 6, 3, 4, 5]

// This means the front of your first sheet of paper
// should have pages 8 and 1 of your document side-by-side,
// then the back should have pages 2 and 7, then the front
// of your second sheet should have pages 6 and 3, etc.

// 0 indicates that a portion should be blank

const createBookletOrder = pages => {
    const source = pages,
          res = [];

    while (pages % 4 !== 0) pages++;

    let up = 1,
        down = pages;

    for (let i = 0; i < pages; i++) {
        const x = i % 4,
              y = (x === 1 || x === 2) ? up++ : down--;
        
        res.push(y > source ? 0 : y);
    }

    return res;
};