One article explains in detail how to implement three-level linkage menu with JS (with explanation of ideas)-JS Tutorial-php.cn

Idea: Each drop-down menu, as a component, can receive a set of data and generate different menu options based on different data contents. The correlation between the three levels is achieved through event throwing. Data is obtained from the background.

When you click the province menu to select Shaanxi, the menu component will throw out the current province through event throwing. After knowing the province, you can obtain the city data under the province from the background. And so on. [Related recommendations: JavaScript video tutorial]

Achievement effect:

Front-end and back-end interface Information:

Three-level cascading menu interface:

## URL: http://10.9.72.245:4010

## Method: "GET"

## Data format:

Request: QueryString

Response: JSON

Interface name:

1. http:// 10.9.72.245:4010/getProvince

2. http://10.9.72.245:4010/getCity

3. http://10.9.72.245:4010/getCounty

Get province interface

Interface name:/getProvince

Request: No parameters

Response: {"province":["Beijing","Tianjin","Hebei" ,...]}

Get city interface:

Interface name:/getCity

Request:?province="Hebei"

Response: { "city":["Shijiazhuang", "Tangshan", "Qinhuangdao",...]}

Get county interface:

Interface name:/getCounty

Request ：?city="Shijiazhuang"

Response: {"county":["Chang'an District", "Qiaodong District", "Qiaoxi District",...]}

Specific implementation :

1. Create drop-down menu component and front-end and back-end communication:

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script type="module">
        import QueryString from &#39;./js/QueryString.js&#39;;
        import DropDownMemu from &#39;./js/DropDownMemu.js&#39;;
 
        let cityInfo = {};
        init();
        function init(){
            ajax("http://10.9.72.245:4010","getProvince").then(succeseFunction).catch(failFunction);
        }
        // ajax通信成功后执行：
        function succeseFunction(_data){
            let data = JSON.parse(_data);
            let key = Object.keys(data)[0];
            data = Object.values(data)[0];
            if(DropDownMemu.Obj[key]){
                DropDownMemu.Obj[key].list = data;
                DropDownMemu.Obj[key].name = data[0];
            }else{
                let memu = new DropDownMemu(key);
                memu.addEventListener("change",changeHandler);
                memu.list = data;
                memu.name =data[0];
                memu.appendTo("body");
                cityInfo[key] = data[0];
            }
        }
        // 当菜单显示内容改变时接收到菜单抛发出的事件并获取事件中携带的信息例如{"province":"陕西"}或者{"city":"西安"}
        function changeHandler(e){
            let key = e.currentTarget.label;
            cityInfo[key] = e.data[key];
            let interfaceName;
            if(e.data.province){
                interfaceName = "getCity";
            }else if(e.data.city){
                interfaceName = "getCounty";
            }else{
                return
            }
            ajax("http://10.9.72.245:4010",interfaceName,cityInfo).then(succeseFunction).catch(failFunction);
        }
        /*
            ajax通信：
            参数列表：
            url: 后台服务地址
            interfaceType：接口类型，如 "getProvince"
            data:传输的数据,例如:{"province":"陕西"}
            通信类型type:默认"GET"请求
            是否以json格式发送数据：默认为false
        */
        function ajax(url,interfaceType,data,type="get",json=false){
            type = type.toUpperCase();
            let o = type==="GET"? null : data;
            if(data) data = json? JSON.stringify(data) : QueryString.stringify(data);
            else data = "";
            return new Promise(function(resolve,reject){
                let xhr = new XMLHttpRequest();
                xhr.open(type,url + "/" + interfaceType + (type==="GET"? "?"+data : ""));
                xhr.send(o);
                xhr.onreadystatechange = function(){
                    if(xhr.readyState===4 && xhr.status===200){
                        resolve(xhr.response);
                    }else if(xhr.readyState===4){
                        resolve(xhr.status);
                    }
                }
                xhr.onerror= function(){
                    reject(xhr.response);
                }
            })
        }
        // ajax通信失败时执行
        function failFunction(_err){
            console.log(_err);
        }
 
 
    </script>
</body>
</html>

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2. Drop-down menu component: DropDownMemu class

import Component from "./Component.js";

export default class DropDownMemu extends Component {

    _list;      // 当前下拉菜单的可选项。
    _name;      // 当前选择显示的名字例如： "北京"
    label;      // 当前下拉菜单的标签，province city county
    spanLabel;  // 标签容器
    spanCaret;  // 三角形
    ul;         // 下拉选项容器
    bool=false; // 控制鼠标事件，聚焦状态或正在选择时，不触发鼠标划入滑出效果。
    
    // 根据不同的状态设置下拉菜单的样式。
    static DROPDOWN = Symbol();
    static DEFAULT = Symbol();
    // 静态全局变量 每创建一个下拉菜单，都存储到这个对象中，全局管理创建的每一个下拉菜单。
    static Obj = {};

    constructor(_label) {
        super("p");
        this.label = _label;
        // 创建HTML结构
        this.render();
        // 设置样式
        this.setStyle();
        // 鼠标滑入滑出点击，聚焦失焦，点击事件
        this.elem.addEventListener("focusin", e =>this.mouseHandler(e));
        this.elem.addEventListener("focusout", e =>this.mouseHandler(e));
        this.elem.addEventListener("mouseenter", e=>this.mouseHandler(e));
        this.elem.addEventListener("mouseleave", e=>this.mouseHandler(e));
        this.elem.addEventListener("click", e => this.mouseHandler(e));
    }
    
    mouseHandler(e){
        switch(e.type){
            case "mouseenter": 
                if(this.bool) return
                this.elem.style.backgroundColor = "#e6e6e6";
                break;
            case "mouseleave":
                if(this.bool) return
                this.elem.style.backgroundColor = "#fff";
                break;
            case "focusin":
                this.setState(DropDownMemu.DROPDOWN);
                this.bool = true;
                break;
            case "focusout":
                this.setState(DropDownMemu.DEFAULT);
                this.bool = false;
            case "click" :
                if(e.target.constructor !== HTMLLIElement) return
                this._name = e.target.textContent;
                // 当点击时修改当前显示的内容，重设样式，并抛发事件告知外部当前的内容。
                this.setContent();
                let evt = new FocusEvent("focusout");
                this.elem.dispatchEvent(evt);
        }
    }
    
    set name(_name){
        this._name = _name;
        this.setContent();
    }
    get name(){
        return this._name;
    }
    set list(_list){
        this._list = _list;
        this.ul.innerHTML = "";
        this.ul.appendChild(this.createLi());
    }
    // 修改菜单当前显示的内容并并抛发数据
    setContent(_name){
        this._name = _name || this._name;
        this.spanLabel.textContent = this._name;
        let evt = new MouseEvent("change");
        if(!evt.data) evt.data = {}
        evt.data[this.label] = this._name;
        this.dispatchEvent(evt);
    }
    // 根据指定的list创建下拉菜单选项。
    createLi(_list){
        this._list = _list || this._list;
        let elem = document.createDocumentFragment();
        this._list.forEach((item, index) => {
            let li = document.createElement("li");
            li.textContent = item;
            Object.assign(li.style, {
                lineHeight:"26px",
                padding:"0 15px",
            })
            elem.appendChild(li);
        })
        return elem;
    }
    setState(type){
        switch(type){
            case DropDownMemu.DROPDOWN:
                this.elem.style.backgroundColor = "#e6e6e6";
                this.ul.style.display = "block";
                break;
            case DropDownMemu.DEFAULT:
                this.elem.style.backgroundColor = "#fff";
                this.ul.style.display = "none";
                break;
        }
    }
    appendTo(parent){
        super.appendTo(parent);
        DropDownMemu.Obj[this.label] = this;
    }
    render() {
        this.elem.setAttribute("tabIndex",1);
        this.spanLabel = document.createElement("span");
        this.spanCaret = document.createElement("span");
        this.ul = document.createElement("ul");
        this.elem.appendChild(this.ul);
        this.spanLabel.textContent = this._name;
        this.elem.appendChild(this.spanLabel);
        this.elem.appendChild(this.spanCaret);
    }
    setStyle() {
        Object.assign(this.elem.style, {
            float: "left",
            minHeight: "20px",
            minWidht: "80px",
            color: "#333",
            fontWeight: "normal",
            textAlign: "center",
            whiteSpace: "nowrap",
            verticalAlign: "middle",
            cursor: "pointer",
            border: "1px solid #ccc",
            borderRadius: "4px",
            backgroundColor: "#fff",
            padding: "6px 12px",
            fontSize: "14px",
            userSelect: "none",
            marginRight: "100px",
            position:"relative",
        });
        Object.assign(this.spanLabel.style, {
            float: "left",
            padding: "0 5px"
        })
        Object.assign(this.spanCaret.style, {
            display: "inline-block",
            verticalAlign: "middle",
            borderTop: "4px dashed",
            borderRight: "4px solid transparent",
            borderLeft: "4px solid transparent",
        })
        Object.assign(this.ul.style, {
            listStyle: "none",
            position: "absolute",
            top: "100%",
            left: "0",
            zIndex: "1000",
            minWidth: "100px",
            padding: "5px 0px",
            margin: "2px 0 0",
            fontSize: "14px",
            textAlign: "left",
            backgroundColor: "#fff",
            border: "1px solid rgba(0, 0, 0, 0.15)",
            borderRadius: "4px",
            boxShadow: "0 6px 12px rgba(0, 0, 0, 0.175)",
            display: "none",
        })
    }
}

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3. Component parent class:

export default class Component extends EventTarget{
    elem;
    constructor(_type){
        super();
        this.elem = this.createElem(_type);
    }
    createElem(_type){
        let elem = document.createElement(_type);
        return elem;
    }
    appendTo(parent){
        if(typeof parent==="string") parent = document.querySelector(parent);
        parent.appendChild(this.elem);
    }
}

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4. Nodejs background service:

let http = require("http");
let querystring = require("querystring");
let data,
    req,
    res;

// 读取所有城市数据并解析为对象，同步读取。
let fs = require("fs");
let allCityInfo = JSON.parse(fs.readFileSync(&#39;./city.json&#39;));

let server = http.createServer(listenerHandler);
server.listen(4010,"10.9.72.245",listenerDoneHandler);

function listenerHandler(_req,_res){
    req = _req;
    res = _res; 
    res.writeHead(200,{
        "content-type":"text/html;charset=utf-8",
        "Access-Control-Allow-Origin":"*",
        "Access-Control-Allow-Headers":"*",
    });
    data="";
    req.on("data",function(_data){
        data=_data;
    })
    req.on("end",receiveHandler);
}
function receiveHandler(){
    // console.log(allCityInfo);
    // 根据请求头的url解析接口类型
    let type = req.url.trim().split("?")[0].replace(/\//g,"");
    console.log(type);
    // 根据请求头的url解析传入的参数
    if(req.method.toUpperCase()==="GET"){
        if(req.url.includes("favicon.ico")) return res.end();
        else data = req.url.includes("?") ? req.url.split("?")[1] : "";
    }
    try{
        data = JSON.parse(data);
    }catch{
        data = querystring.parse(data);
    }
    console.log(data);

    // 根据接口类型查找数据。
    let list = {};
    switch(type){
        case "getProvince":
            list.province = Object.keys(allCityInfo);
            break;
        case "getCity" :
            list.city = Object.keys(allCityInfo[data.province]);
            break;
        case "getCounty":
            list.county = allCityInfo[data.province][data.city];
            break;
    }

    console.log(list);
    res.write(JSON.stringify(list));
    res.end()
}
function listenerDoneHandler(){
    console.log("开启服务成功");
}

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5. Server data is stored in city.json in json format, as shown below:

{

"Beijing": {

"Beijing": ["Dongcheng District", "Xicheng District", "Chongwen District", "Xuanwu District", "Chaoyang District", "Fengtai District", "Shijingshan District", "Haidian District", "Mentougou District", "Fangshan District", "Tongzhou District", "Shunyi District", "Changping District", "Daxing District", "Pinggu District", "Huairou District", "Miyun County", "Yanqing County", "Others"]

},

"Tianjin": {

"Tianjin": ["Heping District", "Hedong District", "Hexi District", "Nankai District", "Hebei District", "Hongjie District", "Binhai New District", "Dongli District", "Xiqing District", "Jinnan District", "Beichen District", "Ninghe District", "Wuqing District", "Jinghai County", "Baodi District", "Jixian", "Tanggu District", "Hangu District" ", "Dagang District", "Baodi District", "Others"]

},

}

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