var a = 10;
var obj1 = {
    a: 20,
    fn: (function() {
        this.a *= 2;
        a = 5;
        var a = 6;
        return function() {
            this.a *= a;
            console.log(a)
        }
    })()
}
var obj2 = {
    a: 30
}
var fn = obj1.fn;
fn();  //6???
obj1.fn();  //6???
obj1.fn.call(obj2);  //6???

as shown above, after the function runs, the result is 6, respectively, but I don"t understand why it is 6. Even if an equals 6 after var a = 6, then why is an equal to 6 after this.a * = a? Ask the great god for advice.