请问回调函数中的this指向怎么判断

为什么objA.fn1()指向window，objA.fn2()指向objA，完全晕了

        function createData1(callback) {
            callback();
            console.log('回调函数createData1')
        }
        
        let objA = {
            count: 1,
            fn1: function () {
                createData1(function () {
                    console.log('fn1_1', this)
                })
            },
            fn2: function () {
                createData1(() => {
                    console.log('fn1_2', this)
                })
            }
        }
        objA.fn1()
        objA.fn2()