matrix function

matrix.js

const last = (arr) => arr.slice(-1)[0];
const init = (arr) => arr.slice(0, -1);
const matrix = (row, fnRow) => (col, fnItem) =>
  [...Array(row * col).keys()].reduce((z, x) => {
    const lastRow = last(z);
    const initRows = init(z);
    const item = fnItem(x);

    if (lastRow.length < col) {
      const rowUpdated = lastRow.concat(item);
      return rowUpdated.length === col ?
        initRows.concat([fnRow(rowUpdated)]) :
        initRows.concat([rowUpdated]);
    }

    return initRows.concat([lastRow], [[item]]);
  }, [[]]);

const matrix4r2c = matrix
  (4, (row) => row.concat(0))
  (2, (item) => item + 2)

console.log(matrix4r2c);
//
// [[2, 3, 0],
//  [4, 5, 0],
//  [6, 7, 0],
//  [8, 9, 0]]
//