Native implementation of common helper functions

utils.js

function toPairs(obj) {
  return Object.entries(obj); // ES2017
}

describe("toPairs", () => {
  it("transforms object to key-value pairs", () => {
    expect(toPairs({ foo: "bar", foz: "baz" })).toEqual([
      ["foo", "bar"],
      ["foz", "baz"]
    ]);
  });
});

function fromPairs(arr) {
  return arr.reduce((akk, [k, v]) => ({ ...akk, [k]: v }), {});
}

describe("fromPairs", () => {
  it("transforms pairs to an object", () => {
    expect(fromPairs([["foo", "bar"], ["foz", "baz"]])).toEqual({
      foo: "bar",
      foz: "baz"
    });
  });
});

function range(start, end) {
  return [...Array(end - start)].map((v, i) => i + start);
}

describe("range", () => {
  it("returns a range of numbers", () => {
    expect(range(1, 5)).toEqual([1, 2, 3, 4]);
    expect(range(0, 4)).toEqual([0, 1, 2, 3]);
    expect(range(-1, 3)).toEqual([-1, 0, 1, 2]);
  });
});