Javascript global variable access

Question

Can someone help me understand why the value of x is 'undefined'?

var x = 100;

function test() {

    if(false) {

        var x = 199;
    }

    alert(x);
}

test();

https://stackoverflow.com/search?q=%5Bjavascript%5D+hoisting — deceze, Aug 10 '17 at 12:49
@litelite The `x` outside of the function gets shadowed by the `x` declared inside the function — Lennholm, Aug 10 '17 at 12:50
Even it is declared and assigned inside the `if(false)` statement Javascript run something called **hoisting**, that pop ups all the variables declared inside a closure in to the beginning of that closure. So the variable is assigned inside the `if` but it is really declared previously. As the code asks for the x value it prints Undefined. The x global variable is not taked in account because there is another with the same name declared inside the closure of the funcion test. Check https://www.w3schools.com/js/js_function_closures.asp and https://www.w3schools.com/js/js_hoisting.asp — jmtalarn, Aug 10 '17 at 13:00

score 0 · Answer 1 · edited Aug 10 '17 at 13:03

0

When you define a new variable inside your if it is visible only inside it.

Considering that you declare again the variable x by writing var before, it become a variable visible only inside the if.

By removing that var, it will be changed if the if verifies and the alert will always work.

var x = 100;

function test() {

    if(false) {

        x = 199;
    }

    //alert(x);
    console.log(x);
}

test();

edited Aug 10 '17 at 13:03

Sagar V

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answered Aug 10 '17 at 12:50

Marco Salerno

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Why not declaring the variable inside the `test()` function? – node_modules Aug 10 '17 at 12:52
2

"a variable visible only inside the `if`" – no, not really. That doesn't explain the behaviour. – deceze Aug 10 '17 at 12:52
@Jer idk it depends from what he needs, i just explained which was the problem ^^ – Marco Salerno Aug 10 '17 at 12:53
@deceze yes really, if you instantiate a new variable inside an if as he did, it will be visible only inside it ^^ – Marco Salerno Aug 10 '17 at 12:54
So why is the alert giving `undefined` instead of `100` or alternatively `ReferenceError`?! – deceze Aug 10 '17 at 12:57
cause he can't see the value cause it has been declared inside the if as i said – Marco Salerno Aug 10 '17 at 12:58
Then why isn't the value `100`? – deceze Aug 10 '17 at 13:01
cause he declared again the variable as a new variable, not visible cause of the scope – Marco Salerno Aug 10 '17 at 13:02
Aha! That's rather missing from your answer. Or at the very least unclear. – deceze Aug 10 '17 at 13:03
My answer looks clear to me, maybe it's cause english isn't my most used language ^^ – Marco Salerno Aug 10 '17 at 13:08
@MarcoSalerno You need to learn the difference between declaring a variable and assigning a value to a variable. It's always two different processes that happens separately, even if JS syntax makes it look like one single process, which is why confusions like this happen. – Lennholm Aug 10 '17 at 13:23
I just know the difference – Marco Salerno Aug 10 '17 at 13:28

Javascript global variable access

1 Answers1