1-7 Test many input combinations using Property-based testing

⚪ ️ 1.7 Test many input combinations using Property-based testing §

:white_check_mark: Do: Typically we choose a few input samples for each test. Even when the input format resembles real-world data (see bullet ‘Don’t foo’), we cover only a few input combinations (method(‘’, true, 1), method(“string” , false , 0)), However, in production, an API that is called with 5 parameters can be invoked with thousands of different permutations, one of them might render our process down (see Fuzz Testing). What if you could write a single test that sends 1000 permutations of different inputs automatically and catches for which input our code fails to return the right response? Property-based testing is a technique that does exactly that: sending all the possible input combinations to your unit under test it increases the serendipity of finding a bug. For example, given a method — addNewProduct(id, name, isDiscount) — the supporting libraries will call this method with many combinations of (number, string, boolean) like (1, “iPhone”, false), (2, “Galaxy”, true). You can run property-based testing using your favorite test runner (Mocha, Jest, etc) using libraries like js-verify or testcheck (much better documentation). Update: Nicolas Dubien suggests in the comments below to checkout fast-check which seems to offer some additional features and also to be actively maintained

❌ Otherwise: Unconsciously, you choose the test inputs that cover only code paths that work well. Unfortunately, this decreases the efficiency of testing as a vehicle to expose bugs

✏ Code Examples

:clap: Doing It Right Example: Testing many input permutations with “fast-check” §

import fc from "fast-check";
 
describe("Product service", () => {
  describe("Adding new", () => {
    //this will run 100 times with different random properties
    it("Add new product with random yet valid properties, always successful", () =>
      fc.assert(
        fc.property(fc.integer(), fc.string(), (id, name) => {
          expect(addNewProduct(id, name).status).toEqual("approved");
        })
      ));
  });
});