VampireFang

document.addEventListener("DOMContentLoaded", function() {
            function generateOptions() {
                return {
                    "AUDI": {
                        "80": {
                            url: "p80.html"
                        },
                        "100": {
                            url: "audi100.html"
                        },
                        "url": "AUDI.html"
                    },
                    "BMW": {
                        "3 series": {
                            url: "p3.html"
                        },
                        "5 series": {
                            url: "p5.html"
                        },
                        "url": "BMW.html"
                    },
                    "url": "All.html"
                };
            }
            const elem = document.querySelector(".chained-selects");
            const selects = Array.from(elem.querySelectorAll("select"));
            const getOptions = (obj, path = []) => Object.keys(path.reduce((obj, key) => obj[key], obj)).filter(key => key !== "url");
            const createOptions = (target, options) => target.append(...options.map(i => new Option(i, i)));
            let options = getOptions(generateOptions());
            let target = selects[0];
            createOptions(target, options);
            const change = ({
                target
            }) => {
                let disabled = false,
                    path = [],
                    last;
                for (let select of selects) {
                    let value = select.value;
                    select.disabled = disabled;
                    if (disabled) select.options.length = 1;
                    if (target === select) {
                        last = true;
                    } else if (!disabled && last) {
                        select.options.length = 1;
                        let options = getOptions(generateOptions(), path);
                        createOptions(select, options);
                    };
                    if (select.selectedIndex === 0) disabled = true;
                    else path.push(value);
                }
            }
            elem.addEventListener("change", change);
            elem.querySelector("button").addEventListener("click", () => {
                let obj = generateOptions();
                for (let {
                        value
                    } of selects) {
                    if (value in obj) obj = obj[value];
                    else {
                        if ("url" in obj) break;
                        value = Object.keys(obj)[0];
                        obj = obj[value];
                    }
                }
                location.href = obj.url
            });
        })