以下代码输出什么？解释原因。

var name = 'window'
var obj = {
  name: 'obj',
  fn1: function () { console.log(this.name) },
  fn2: () => console.log(this.name),
  fn3: function () {
    return function () { console.log(this.name) }
  },
  fn4: function () {
    return () => console.log(this.name)
  }
}

obj.fn1()        // 'obj'
obj.fn2()        // 'window'
obj.fn1()()      // ❌ fn1 不返回函数，这里报错
var f3 = obj.fn3()
f3()             // 'window'
var f4 = obj.fn4()
f4()             // 'obj'