JS验证身份证的合法性[通俗易懂]

//验证身份证的合法性
function IdentityCodeValid(code) { 
            var city={11:"北京",12:"天津",13:"河北",14:"山西",15:"内蒙古",21:"辽宁",22:"吉林",23:"黑龙江 ",31:"上海",32:"江苏",33:"浙江",34:"安徽",35:"福建",36:"江西",37:"山东",41:"河南",42:"湖北 ",43:"湖南",44:"广东",45:"广西",46:"海南",50:"重庆",51:"四川",52:"贵州",53:"云南",54:"西藏 ",61:"陕西",62:"甘肃",63:"青海",64:"宁夏",65:"新疆",71:"台湾",81:"香港",82:"澳门",91:"国外 "};
            var tip = "";
            var pass= true;
            
            if(!code || !/^\d{6}(18|19|20)?\d{2}(0[1-9]|1[12])(0[1-9]|[12]\d|3[01])\d{3}(\d|X)$/i.test(code)){
                tip = "身份证号格式错误";
                pass = false;
            }
           else if(!city[code.substr(0,2)]){
                tip = "地址编码错误";
                pass = false;
            }
            else{
                //18位身份证须要验证最后一位校验位
                if(code.length == 18){
                    code = code.split('');
                    //∑(ai×Wi)(mod 11)
                    //加权因子
                    var factor = [ 7, 9, 10, 5, 8, 4, 2, 1, 6, 3, 7, 9, 10, 5, 8, 4, 2 ];
                    //校验位
                    var parity = [ 1, 0, 'X', 9, 8, 7, 6, 5, 4, 3, 2 ];
                    var sum = 0;
                    var ai = 0;
                    var wi = 0;
                    for (var i = 0; i < 17; i++)
                    {
                        ai = code[i];
                        wi = factor[i];
                        sum += ai * wi;
                    }
                    var last = parity[sum % 11];
                    if(parity[sum % 11] != code[17]){
                        tip = "校验位错误";
                        pass =false;
                    }
                }
            }
            return pass;
        }