JavaScript和VBScript函数调用

Question

HTML Form:

<form 
    name="CheckIn" 
    method="post" 
    action="check.asp" 
    onsubmit="return CheckBreakTime()  && CheckTime();" 
>

I call two functions whit onsubmit,the functions return true or false,but function works only if it's "first" ,for example if i say ' onsubmit="return CheckTime() && CheckBreakTime();" ' only CheckTime works if i call ChecKBreakTime first it only works.

The Functions (JavaScript) :

function CheckBreakTime(){
    if (document.getElementById('breakco').checked) {
        var BKTimeDif1 = '<%=BTimeDif%>';
        var var1 = 20 ;
        var sum1 = var1 - BKTimeDif1 ;
        if (BKTimeDif1 > 10 && BKTimeDif1 < 21) {
            alert("You were on a break longer than 10 minutes,You must wait " + sum1 + " minutes to pass to check out from break. ");
            return false;
        } else {
            return true;
        }
    } 

    else {
        return true;  
        }
    }

function CheckTime() {
    if (document.getElementById('breakci').checked) {
        var TimeDif2 = '<%=BTimeDiff%>';
        var TimeDif1 = '<%=TimeDif%>';
        if (TimeDif1 < 120) {
            alert("You must work at least two hours before you can go on a break.");
            return false;
        } else {
            if (TimeDif2 != 0 && TimeDif2 < 120) {
                alert("You must work at least two hours before you can go on another break.");
                return false;
            }
            else {
                return true;
            }
        }


    }
    else {
        return true;
    }
}

and the VBScript code that i put in JavaScript:

Dim TimeDif
TimeDif=Round(Csng(DateDiff("n", (HourList.Fields.Item("checkInTime").Value), (Now()))), 2)
Dim BTimeDif
If Not IsNull(HourList.Fields.Item("breakCheckIn").Value) Then
BTimeDif = Round(Csng(DateDiff("n", (HourList.Fields.Item("breakCheckIn").Value), (Now()))), 2)
End If
If Not IsNull(HourList.Fields.Item("breakCheckOut").Value) Then    
Dim BTimeDiff
BTimeDiff = Round(Csng(DateDiff("n", (HourList.Fields.Item("breakCheckOut").Value), (Now()))), 2)
End If

VBScript code works fine,it returns what it need to and JavaScript gets it.Can some tell me what is the problem...

Answer 1

JavaScript uses short-circuit evaluation for its logical operators. That means that in an expression like expr1 && expr2 , expr2 is evaluated only if expr1 evaluates to true. If expr1 evaluates to false there is no need to evaluate expr2 , since the result of expr1 && expr2 will be to false no matter what.

So, if you run for example

var result = foo() && bar();

function foo(){
  console.log("running foo!");
  return false;
}

function bar(){
  console.log("running bar!");
  return false;
}

Only function foo() will be executed. This is effectively the same as:

var result = foo();
if (result){
  result = bar();
}

That's why only the first function in your expression gets called. If it returns false, there's no need to run the second one.

Answer 2

If you want both functions to run even if the first one evaluates to false - then you can use "&": onsubmit multiple javascript functions

But, personally I'd wrap them both in a function that aggregates the logic of both:

function checkBothTimes(){
  if (!CheckBreakTime()){
    return false;
  }
  else if (!CheckTime()){
    return false;
  }
  return true;
}

Even though this is longer - it makes the code easier to read (to me).