Why the result is NaN?

JavaScript
1 
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.in sayHi() function, there is a line var a; i don’t know why the a is undefined。 thank you

2

Because a is defined in the global scope and can’t be reached from within your function. If you want the function to use that variable you need to pass it as a parameter, like so


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

Basically you now have two different a variables; one lives within the global scope while to other lives only within the function sayHi. This is probably easier to understand if the function uses a different variable name:


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

This is exactly the same as the above but I now use a different variable within the function; semantically they’re the same thing.

You may want to read up on variable scopes in javascript. For example http://coding.smashingmagazine.com/2009/08/01/what-you-need-to-know-about-javascript-scope/