1.

var a = 10;
function test() {
    a = 100;
    console.log(a);
    console.log(this.a);
    var a;
    console.log(a);
}
test();//100  10  100

=>

var a = 10;//全域性
function test() {
    var a;//變數提升
    a = 100;//區域性
    console.log(a);//區域性
    console.log(this.a);//this指向全域性window
    console.log(a);//區域性
}
test();
 
//100  10  100

2.

var a = 100;
function test() {
    console.log(a);
    var a = 10;
    console.log(a);
}
test();//undefined  10

=>

var a = 100;
function test() {
    var a;//變數提升
    console.log(a);
    a = 10;
    console.log(a);
}
test();//undefined  10

3.

var a = 100;
function test() {
    console.log(a);
    a 
 
= 10;
    console.log(a);
}
test();
console.log(a);
//100  10  10

=>

var a = 100;
function test() {
    console.log(a);//全域性
    a = 10;//全域性
    console.log(a);//全域性
}
test();
console.log(a);
//100  10  10