How to understand global and local variable in javascript [duplicate]

Question

1st test:

var a = 1;
function b() {
    a = 10;
    return;
    function a() {}
}
b();
alert(a);    // 1

2nd test:

var a = 1;
function b() {
    a = 10;
    return;
}
b();
alert(a);    // 10

In the first test, a is equal to 1, although I set it to 10 in the method. In the second test, I set it to 10 and it is set to 10 when I output it.. How does this work?

Answer 1

The function declaration function a() {} declares a variable name a in the local scope of your b function (and assigns it the function). When you are assigning to a, you will assign to that local variable not the global one.

With hoisting applied, your code is equivalent to

var b = function b() {
    var a = function a() {};
    a = 10;
    return;
}
var a = 1;
b();
alert(a);    // 1, obvious now