复制
1function Foo(){
2    Foo.a = function(){
3        console.log(1);
4    }
5    this.a = function(){
6        console.log(2)
7    }
8}
9
10Foo.prototype.a = function(){
11    console.log(3);
12}
13
14Foo.a = function(){
15    console.log(4);
16}
17
18Foo.a();
19let obj = new Foo();
20obj.a();
21Foo.a();