Javascript functions question, could someone possibly help explain?

Question

I am currently trying to learn Javascript and understand that functions are one of the most important aspects of the language but I have to say Im finding it very hard to understand them especially when different parameters are being used here there and everywhere.

I have been looking at this code which I read in another stack overflow post regarding closure and cant understand how 16 is alerted, I have used console.log to work out what value is being used at each point and seem to get a total of 17, could someone possibly explain?

function foo(x) {
 // console.log(x); = 2    
 var tmp = 3;
 return function (y) {
  // console.log(y); = 10
  alert(x + y + (++tmp));
  // console.log(++tmp); = 5?
 }
}
var bar = foo(2);
bar(10);

If anyone can offer any wisdom about functions that might make things start to make sense for me it would be really appreciated.

Kyle

Ondřej Mirtes · Accepted Answer · 2011-07-23 13:42:47Z

5

The alert() command really pops up 16.

++tmp is preincrement statement - first, it increments the value (from 3 to 4) and then continues with execution of the line.

So 2 + 10 + 4 is passed to the alert().

The console.log(++tmp) below the alert logs 5, because the tmp variable is incremented again, from 4 to 5.

answered Jul 23, 2011 at 13:42

Ondřej Mirtes

5,85128 silver badges39 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

ChristopheCVB · Accepted Answer · 2011-07-23 13:42:48Z

0

Try to log

console.log(tmp);

Instead of

console.log(++tmp);

answered Jul 23, 2011 at 13:42

ChristopheCVB

7,3031 gold badge31 silver badges54 bronze badges

Comments

Kon · Accepted Answer · 2011-07-23 13:43:13Z

0

foo(2) initializes tmp to 3 and x to 2 within the scope of the returned function. bar(10) then adds 2, 10, and the result of incremented tmp (which went from 3 to 4 before being added to x and y).

so the alerted value will be the sum of 2 + 10 + 4.. 16.

answered Jul 23, 2011 at 13:43

Kon

27.5k12 gold badges64 silver badges87 bronze badges

Collectives™ on Stack Overflow

Javascript functions question, could someone possibly help explain?

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related