pruncalc/node_modules/d3-array/src/cross.js

function length(array) {
  return array.length | 0;
}

function empty(length) {
  return !(length > 0);
}

function arrayify(values) {
  return typeof values !== "object" || "length" in values ? values : Array.from(values);
}

function reducer(reduce) {
  return values => reduce(...values);
}

export default function cross(...values) {
  const reduce = typeof values[values.length - 1] === "function" && reducer(values.pop());
  values = values.map(arrayify);
  const lengths = values.map(length);
  const j = values.length - 1;
  const index = new Array(j + 1).fill(0);
  const product = [];
  if (j < 0 || lengths.some(empty)) return product;
  while (true) {
    product.push(index.map((j, i) => values[i][j]));
    let i = j;
    while (++index[i] === lengths[i]) {
      if (i === 0) return reduce ? product.map(reduce) : product;
      index[i--] = 0;
    }
  }
}