main.js

var num = 0,result = 0,numshow = "0"; 
var operate = 0; //determine input state
var calcul = 0; //determine calculator state
var quit = 0; //avoid click twice

function command(num){
    var str=String(document.calculator.numScreen.value); //get value
    str = (str !="0") ? ((operate==0) ? str:"") :""; // if value is != "0" but state is 0, return value; or return null; 
    str = str + String(num); //add character
    document.calculator.numScreen.value = str; //refresh
    operate=0; // reset input
    quit=0; // reset avoid click twice
}

function dzero(){
    var str=String(document.calculator.numScreen.value);
    str = (str != "0") ? ((operate==0) ? str+"00":"0") : "0";
    document.calculator.numScreen.value = str;
    operate = 0;
}

function dot(){
    var str=String(document.calculator.numScreen.value);
    str = (str != "0") ? ((operate==0) ? str : "0") : "0";
    for(i=0;i<=str.length;i++){ //determine whether a dot is existing
        if(str.substr(i,1)==".") return false; 
    }
    str = str + "."
    document.calculator.numScreen.value=str;
    operate=0;
}

function del(){ 
    var str=String(document.calculator.numScreen.value);
    str = (str != "0") ? str : "";
    str = str.substr(0,str.length-1)
    str = (str != "") ? str : "0";
    document.calculator.numScreen.value=str;
}

function clearscreen(){
    num=0;
    result=0;
    numshow="0";
    document.calculator.numScreen.value="0";//原文里是等于str 0
}

//operation
function plus(){
    calculate();
    operate=1;
    calcul=1;
}

function minus(){
    calculate();
    operate=1;
    calcul=2;
}

function times(){
    calculate();
    operate=1;
    calcul=3;
}

function divide(){
    calculate();
    operate=1;
    calcul=4;
}

function persent(){
    calculate();
    operate=1;
    calcul=5;
}

function equal(){
    calculate();
    operate=1;
    num=0;
    result=0;
    numshow="0";
}

//calculate
function calculate(){
    numshow=Number(document.calculator.numScreen.value);
    if (num != 0 && quit != 1){
        switch(calcul){
            case 1: result = num + numshow;break;
            case 2: result = num - numshow;break;
            case 3: result = num * numshow;break;
            case 4:if(numshow != 0){result = num/numshow;} else{document.getElementById("note").innerHTML="被除数不能为零！"; setTimeout(clearnote,4000)} break;
            case 5: result = num & numshow;break;
        }
        quit = 1;
    }
    else{
        result = numshow;
    }
    numshow=String(result)
    document.calculator.numScreen.value=numshow;
    num=result;
}

// clear alert
function clearnote(){
    document.getElementById("note").innerHTML="";
}