I've declared the global variable but can't reassign it using the function

Question

Here is the code:

var num = 3

myFunction(num)

function myFunction(aNumber)
{
    aNumber = 20
}

console.log(num)

it still says that 'num' is 3 and I was wondering why?

JS doesn't have pointers. You're assigning `20` to the `aNumber` parameter, which received the current value of `num`, but is otherwise unrelated to `num`. — , Apr 17 '17 at 13:31
`myFunction(num)` *reads* the value 3 from `num` and passes the value (not the variable) into `myFunction`. `myFunction` receives that value in the parameter `aNumber`. `aNumber = 20` stores the value 20 in `aNumber`, which has no effect whatsoever on `num`, since the function doesn't know anything about `num` from the call (it does via other means, but that's nothing to do with the `aNumber` parameter). — T.J. Crowder, Apr 17 '17 at 13:35
`myFunction` *can* update `num`, by doing `num = 20` directly, because `myFunction` is a closure over the context in which `num` is defined. If it *weren't* a closure over the context where `num` is defined, it wouldn't be able to update it directly. — T.J. Crowder, Apr 17 '17 at 13:35
The *usual* way to update a variable via a function is to have the function return the new value: `num = myFunction(num)` and `function myFunction(aNumber) { return aNumber + 10; }` for instance, which adds 10 to the value passed in and returns that new value. In the above, that would end up returning 13, which would then be assigned to `num`. — T.J. Crowder, Apr 17 '17 at 13:36

score 0 · Answer 1 · answered Apr 17 '17 at 13:33

0

You dont have to pass global variables into functions to reassign them.

function myFunction() { 
  num = 20
}
console.log(num);

answered Apr 17 '17 at 13:33

Dan Philip Bejoy

True, but this assumes that the function always wants to mutate the `num` variable. Given the original code, it would be able to mutate any variable passed in (if it worked that way). – Apr 17 '17 at 13:39

1 Answers1