chunkify.js

// alternative version using ES6 Array.from

const chunkify = (chunkSize, src) => Array.from(
  { length: Math.ceil(src.length/chunkSize) },
  (_, i) => src.slice(i * chunkSize, i * chunkSize + chunkSize)
)

unflatten.js

/**
 * "unflatten" / split array in chunks of given size
 */

function unflatten(src, chunkSize) {
    var out = [];
    var chunk = [];
    for (var i = 0; i < src.length; i++) {
        chunk.push(src[i]);
        if (chunk.length === chunkSize || (i + 1) === src.length) {
            out.push(chunk);
            chunk = [];
        }
    }
    return out;
}

var parts = unflatten([1,2,3,4,5,6, 7], 2);
// parts === [ [1, 2], [3, 4], [5, 6], [7] ]

// definition
// flatten(unflatten(someArray, someChunksize)) === someArray