Evenly interleave two arrays

index.test.js

Array.prototype.chunk = function (groupsize) {
  var sets = [];
  var chunks = this.length / groupsize;
  for (var i = 0, j = 0; i < chunks; i++, j += groupsize) {
    sets[i] = this.slice(j, j + groupsize);
  }
  return sets;
};

var interleave = (a1, a2) => {
  if (a1.length < a2.length) {
    return interleave(a2, a1);
  } else if (a1.length > a2.length) {
    const chunksCount = a2.length + 1;
    const chunksSize = Math.ceil(a1.length / chunksCount);
    return a1
      .chunk(chunksSize)
      .map((v, i) => (a2.length === i ? v : [...v, a2[i]]))
      .flat();
  } else {
    return a1.map((v, i) => [v, a2[i]]).flat();
  }
};

describe("interleave", () => {
  it("even input", () => {
    const result = interleave(
      [1, 2, 3, 4, 5, 6],
      ["a", "b", "c", "d", "e", "f"]
    );
    expect(result).toEqual([1, "a", 2, "b", 3, "c", 4, "d", 5, "e", 6, "f"]);
  });
  it("zero length", () => {
    const result = interleave([1, 2, 3, 4, 5, 6], []);
    expect(result).toEqual([1, 2, 3, 4, 5, 6]);
  });
  it("in between", () => {
    const result = interleave([1, 2, 3, 4, 5, 6], ["a", "b"]);
    expect(result).toEqual([1, 2, "a", 3, 4, "b", 5, 6]);
  });
  it("in between just one element", () => {
    const result = interleave([1, 2, 3, 4, 5, 6], ["a"]);
    expect(result).toEqual([1, 2, 3, "a", 4, 5, 6]);
  });
  it("unbalanced interleave", () => {
    const result = interleave([1, 2, 3, 4, 5, 6, 7], ["a", "b"]);
    expect(result).toEqual([1, 2, 3, "a", 4, 5, 6, "b", 7]);
  });
  it("unbalanced interleave one element", () => {
    const result = interleave([1, 2, 3, 4, 5, 6, 7], ["a"]);
    expect(result).toEqual([1, 2, 3, 4, "a", 5, 6, 7]);
  });
  it("more unbalanced interleave", () => {
    const result = interleave([1, 2, 3, 4, 5, 6, 7, 8], ["a", "b"]);
    expect(result).toEqual([1, 2, 3, "a", 4, 5, 6, "b", 7, 8]);
  });
});