JS如何实现简易计算器

<script type="text/javascript">
 // 加法运算
 function plus(arrPlus){
 var sum = 0;
 for(var i = 0 ; i < arrPlus.length ; i++){
 sum = sum + arrPlus[i];
 }
 console.log('函数plus循环后求和结果的数是'+ sum);
 return sum;

 }
 // 减法运算
 function subtract(arrSubtract){
 var substractResult = arrSubtract[0];
 for(var i = 1 ; i < arrSubtract.length ; i++){
 substractResult = substractResult - arrSubtract[i] ;
 }
 return substractResult;
 }
 // 乘法运算
 function multiple(arrMultiple){
 var multipleResult = 1;
 for(var i = 0 ; i < arrMultiple.length ; i++){
 multipleResult = multipleResult * arrMultiple[i];
 }
 return multipleResult;
 }
 // 除法运算
 function divide(arrDivide){
 var divideResult = arrDivide[0];
 for(var i = 1 ; i < arrDivide.length ; i++){
 divideResult = divideResult / arrDivide[i];
 }
 return divideResult; 
 }
 // 输入需要运算的数值
 function calculateNum(num,mention){
 i = 0;
 num = [];
 do{
 // var i = 0;
 mention = prompt('请输入第'+ (i+1) +'个数，注意：'+'输入"计算"二字将获得结果');
 if(mention != '计算'){
 mention = mention * 1;
 num[i]= mention ;
 i++;
 } 
 }while(mention != '计算');
 console.log('calculateNum存入的数值是' + num);
 return num;
 }
 //显示计算表达式
 function formula( arr){
 fom = [] ;
 for(var i =0 ; i < arr.length ; i++){
 if(apply == 1){
 fom = fom + arr[i] + '+';
 }else if (apply == 2){
 fom = fom + arr[i] + '-';
 }else if (apply == 3){
 fom = fom + arr[i] + '*';
 }else if(apply == 4){
 fom = fom + arr[i] + '/';
 } 
 }
 console.log('计算表达式formula函数输出的是' + fom);
 return fom;
 }

 //全局变量
 var i = 0 ;
 var fom = '';
 var num = [];
 var mention = ''
 apply = '';
 do{
 // 计算器主面板
 apply = prompt('欢迎使用简易计算器\n'+'1、加法运算\n'+'2、减法运算\n'+'3、乘法运算\n'+'4、除法运算\n'+'5、退出\n'+'请输入您的选项');
 apply = apply * 1;
 switch(apply){
 case 1:
 var plusInput = calculateNum(num,mention); 
 alert(formula(plusInput)+'计算结果是' + plus(plusInput));
 break;
 case 2:
 var subtractInput = calculateNum(num,mention); 
 alert(formula(subtractInput)+'计算结果是' + subtract(subtractInput));
 break; 
 case 3:
 var multipleInput = calculateNum(num,mention); 
 alert(formula(multipleInput)+'计算结果是' + multiple(multipleInput));
 break;
 case 4:
 var divideInput = calculateNum(num,mention); 
 alert(formula(divideInput)+'计算结果是' + divide(divideInput));
 break; 
 }
 }while(apply != 5)
 alert('欢迎使用！');
</script>