Variable hoisting - “var” with global variable name in function

问题

I was practicing some scenario and find a case:

Here is fiddle

According to closure bar function should have access to var x so I expected to alert 1 and condition get false due to if(!1) but it alerted undefined and condition get true and second alert is with value 10.

var x = 1;
function bar() {
    alert(x);
    if (!x) {
        var x = 10;
    }
    alert(x);
}
bar();

So I am confused why it is prompting undefined?

According to hoisting in a particular scope you define a variable anywhere it is considered as defined at top always.

If it is due to hoisting effect it still have to alert 10 instead of undefined.

回答1:

Hoisting causes a variable to be declared everywhere in the function, not defined.

On the first line of bar, since there is var x on line 3 of the function, the global x is masked and you see the local x (which is undefined since it hasn't been given a value yet).

On line 3 of bar, you have x = 10 which defines the variable. This is not hoisted.

On line 5, you alert it, and it is now defined.

回答2:

Hoisting will make your code effectively work like this:

var x;
x = 1;
function bar() {
    var x; //same as var x = undefined;
    alert(x);
    if (!x) {
        x = 10;
    }
    alert(x);
}
bar();

来源：https://stackoverflow.com/questions/21096904/variable-hoisting-var-with-global-variable-name-in-function