Collate Pages

index.js

function collate(pages = []){
  const flats = [];
  const { length } = pages;
  const flatLength = Math.ceil(length/2);
  const even = length % 2 === 0;
  const newPages = [...pages];
  
  if(!even){
    const lastPage = newPages.pop();
    
    newPages.push(undefined);
    newPages.push(lastPage);
  }
  
  for(let i = 0; i < flatLength; i++){
    const evenFlat = i % 2 === 0;
    
    if(!evenFlat){
      flats.push([
        newPages[i],
        newPages[(newPages.length - 1) - (i)]
      ]);
    }else{
      flats.push([
        newPages[(newPages.length - 1) - (i)],
        newPages[i]
      ]);
    }
  }
  
  return flats;
}

const booklet = collate([
  1, 2, 3, 4, 5, 6, 7, 8, 9
]);

console.log(booklet);

Output:

[
  [
    9,
    1
  ],
  [
    2,
    undefined
  ],
  [
    8,
    3
  ],
  [
    4,
    7
  ],
  [
    6,
    5
  ]
]