SOURCE

function Foo() {
    Foo.a = function () { console.log(1) }
    this.a = function () { console.log(2) }
}
Foo.prototype.a = function () { console.log(3) }
Foo.a = function () { console.log(4) }

Foo.a();
let obj = new Foo();
obj.a();
Foo.a();