Single concept per test

Bad:

import assert from "assert";

describe("MomentJS", () => {
  it("handles date boundaries", () => {
    let date;

    date = new MomentJS("1/1/2015");
    date.addDays(30);
    assert.equal("1/31/2015", date);

    date = new MomentJS("2/1/2016");
    date.addDays(28);
    assert.equal("02/29/2016", date);

    date = new MomentJS("2/1/2015");
    date.addDays(28);
    assert.equal("03/01/2015", date);
  });
});

Good:

import assert from "assert";

describe("MomentJS", () => {
  it("handles 30-day months", () => {
    const date = new MomentJS("1/1/2015");
    date.addDays(30);
    assert.equal("1/31/2015", date);
  });

  it("handles leap year", () => {
    const date = new MomentJS("2/1/2016");
    date.addDays(28);
    assert.equal("02/29/2016", date);
  });

  it("handles non-leap year", () => {
    const date = new MomentJS("2/1/2015");
    date.addDays(28);
    assert.equal("03/01/2015", date);
  });
});