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I use the prototype chain to write a method of linking provinces and municipalities. I put the variables into the initial function so that I can use the prototype chain to expand. There was no problem during initialization, but something went wrong when I used onchange to select a province or city. The attribute options of selectProvince and selectCity could not be found. I looked up and found that selectProvince and selectCity were not detected at all when using onchange. Yeah, I obviously initialized it in AddressInit, but it wasn't detected. Could you please give me a solution?
Code:
//This is a short piece of json

var provinceList = [{
    name: '北京',
    cityList: [{
        name: '市轄區(qū)',
        areaList: ['東城區(qū)', '西城區(qū)', '崇文區(qū)', '宣武區(qū)', '朝陽區(qū)', '豐臺區(qū)', '石景山區(qū)', '海淀區(qū)', '門頭溝區(qū)', '房山區(qū)', '通州區(qū)', '順義區(qū)', '昌平區(qū)', '大興區(qū)', '懷柔區(qū)', '平谷區(qū)']
    }, {
        name: '縣',
        areaList: ['密云縣', '延慶縣']
    }]
}];

//This is html

<select class="select-input" name="selectProvince" id="selectProvince"></select>
    <select class="select-input" name="selectCity" id="selectCity"></select>
    <select class="select-input" name="selectArea" id="selectArea"></select>

//This is js

var AddressInit = function(selectProvince, selectCity, selectArea, defaultProvince, defaultCity, defaultArea){
    this.selectProvince = document.getElementById(selectProvince);
    this.selectCity = document.getElementById(selectCity);
    this.selectArea = document.getElementById(selectArea);
    this.defaultProvince = defaultProvince || '';
    this.defaultCity = defaultCity || '';
    this.defaultArea = defaultArea || '';
}

AddressInit.prototype = {
    constructor: AddressInit,
    //初始化
    init: function(){
        for (var i = 0; i < provinceList.length; i++) {
            this.selectAddOption(this.selectProvince, provinceList[i].name, provinceList[i]);
        }
        this.selected(this.selectProvince, this.defaultProvince);
        this.changeProvince();
        this.selectProvince.onchange = this.changeProvince;
    },
    //判斷是否有選中的值
    selected: function(select,str){
        for(var i = 0; i < select.options.length; i++) {
            if (select.options[i].value == str) {
                select.selectedIndex = i;
                return;
            }
        }
    },
    //添加option
    selectAddOption: function(select, str, obj) {
        var option = document.createElement("option");
            select.options.add(option);
            option.innerText = str;
            option.value = str;
            option.obj = obj;
    },
    //改變省
    changeProvince: function(){
        console.log(selectCity);
        this.selectCity.options.length = 0;
        this.selectCity.onchange = null;
        if (this.selectProvince.selectedIndex == -1) return;
        var item = this.selectProvince.options[this.selectProvince.selectedIndex].obj;
        for (var i = 0; i < item.cityList.length; i++) {
            this.selectAddOption(this.selectCity, item.cityList[i].name, item.cityList[i]);
        }
        this.selected(this.selectCity, this.defaultCity);
        this.changeCity();
        this.selectCity.onchange = this.changeCity;
    },
    //改變市
    changeCity: function(){
        this.selectArea.options.length = 0;
        if (this.selectCity.selectedIndex == -1) return;
        var item = this.selectCity.options[this.selectCity.selectedIndex].obj;
        for (var i = 0; i < item.areaList.length; i++) {
            this.selectAddOption(this.selectArea, item.areaList[i], null);
        }
        this.selected(this.selectArea, this.defaultArea);
    }
}

var ccc = new AddressInit('selectProvince', 'selectCity', 'selectArea');
ccc.init();