Question

var a = 10;
sayHi();
function sayHi()
{
var a = a + 10;
alert(a);
return a;
}
alert(a);
alert(sayHi()+10);

why the above result is not 20 and 30? i feel the first is 20 , then 30.

Answer 1

function sayHi()
{
   var a = a + 10;
   alert(a);
   return a;
}

is same as

function sayHi()
{
    var a;
    // a in this function will be the loacal variable a, which is undefined at first
    // a is undefined, undefined + 10 is NaN
    a = a + 10;
    alert(a);
    return a;
}

Check this article: JavaScript Scoping and Hoisting

If you want to use the global var a, you should not use var a inside the function.

But the better solution is to use the parameter.

function sayHi(a)
{
   a = a + 10;
   alert(a);
   return a;
}