Property based testing Fast-check Javascript

So basically I want to test this function I made. You type in a number "normal_cost" and then in returns 0.2x this value.

const saver = Object.create(null);
const change = (id) => document.getElementById(id);
saver.init = function () {

    change("x").onclick = function multiply() {
        var a, result;
        a = document.getElementById("normal_cost").value;
        result = Math.round((a * 0.2) * 100) / 100;
        document.getElementById("r").innerHTML = result
        };
};
export default Object.freeze(saver);

This is the fast-check test I made but I am not sure what to write after 'return' in the test file below.. Any help would be brilliant thanks!

import multiply from "./saver.js";
import result from "./saver.js";
import a from "./saver.js";


const describe = window.describe;
const it = window.it;
const fc = window.fastcheck;
const chai = window.chai;

const arbNumber = fc.nat(1000);

describe("Property Based Testing", function () {
    it(
        "Given a number;" +
        "after performing a move;" +
        "Score is 0.2 times the number before", function () {
            fc.assert(fc.property(
                arbNumber,

                function(normal_cost) {
                    return multiply.result(normal_cost) === 0.2 * multiply.a(normal_cost);
            }
            ),{"verbose":true})}
    )
});