Propertyp based testing in JavaScript

JSVerify.js

const jsc = require("jsverify");
const _ = require("lodash");

const add = (a, b) => a + b;
const mult = (a, b) => a * b;
const sub = (a, b) => a - b;

const addIsCommunicative = jsc.checkForall(
  jsc.integer,
  jsc.integer,
  (a, b) => add(a, b) === add(b, a)
);

const multIsDistributive = jsc.checkForall(
  jsc.integer,
  jsc.integer,
  jsc.integer,
  (a, b, c) => mult(mult(a, b), c) === mult(a, mult(b, c) + 1)
);

const subsIsCommunicative = jsc.checkForall(
  jsc.integer,
  jsc.integer,
  (a, b) => sub(a, b) === sub(b, a)
);

console.log({ addIsCommunicative, multIsDistributive, subsIsCommunicative });

const emailGenerator = jsc.asciinestring.generator.map(s => `${s}@epost.com`);

const user = jsc.record({
  first: jsc.asciistring,
  last: jsc.asciistring,
  email: jsc.bless({ generator: emailGenerator })
});

jsc.checkForall(jsc.integer, user, (a, u) => {
  console.log({ a, u });
  return !u.email.startsWith("\\");
});

const mittÄrende = {
  delkrav: [
    { typ: "inkasso", summa: 123 },
    { typ: "kapital", summa: 321 },
    { typ: "kapital", summa: 456 },
    { typ: "kapital", summa: 645 }
  ],
  processkostnad: 789
};

const räknaSammanDelkrav = delkrav =>
  delkrav.delkrav.reduce((a, d) => a + d.summa, 0) + delkrav.processkostnad;

const sättNerdelkrav = (delkrav, nr, belopp) => {
  let d = _.cloneDeep(delkrav);
  if (d.delkrav[nr].typ === "kapital") {
    d.delkrav[nr].summa -= belopp;
    d.delkrav[nr].typ = "nedsatt kapital";
  }

  if (d.delkrav[nr].typ === "inkasso") {
    d.delkrav[nr].typ = "nedsatt inkasso";
  }

  return d;
};

const nedsatt = sättNerdelkrav(mittÄrende, 1, 100);

const nedsättningMinskarSumman = jsc.checkForall(
  jsc.integer(0, 3),
  jsc.integer(1, 100),
  (nr, belopp) => {
    var nedsatt = sättNerdelkrav(mittÄrende, nr, belopp);

    return räknaSammanDelkrav(mittÄrende) >= räknaSammanDelkrav(nedsatt);
  }
);