Mastering JavaScript Functional Programming - Second Edition

Testing pure functions

Given the characteristics of pure functions that we have already described, most of your unit tests could simply be the following:

• Call the function with a given set of arguments.
• Verify that the results match what you expected.

Let's start with a couple of simple examples. Testing the isOldEnough() function would have been more complex than we needed for the version that required access to a global variable. On the other hand, the last version, isOldEnough3(), which didn't require anything because it received two parameters, is simple to test: 

describe("isOldEnough", function() {  it("is false for people younger than 18", () => {    expect(isOldEnough3(1978, 1963)).toBe(false);  });    it("is true for people older than ...