JavaScript Flatten Implementation

flatten.js

export default function flatten (arr) {
  return arr.reduce((result, current) => {
    let items;
    if (Array.isArray(current)) {
      items = flatten(current);
      } else if (typeof current === "number") {
      items = [current];
    } else {
      throw new Error(`Expected item to be number or array: ${current}`);
    }
    return result.concat(items);
  }, []);
};

flatten.test.js

import flatten from "./flatten";

describe("flatten function", function() {
  it("is already flat", function() {
    expect(flatten([1, 2, 3, 4])).toEqual([1, 2, 3, 4]);
  });

  it("should flatten nested", function() {
    expect(flatten([[1,2,[3]],4])).toEqual([1, 2, 3, 4]);
  });

  it("should not accept string", function() {
    expect(flatten([1, 2, "a"])).toThrow(
      new Error("Expected item to be number or array: a")
    );
  });
});