怎样在数组处理方法中使用this

var obj = {
    arr: [1,2,3],
    powerArr: function(){
        var self = this;
        self.arr = self.arr.map(function (item) {
            console.log(self);
            return item**2;
        });
    }
};

obj.arr; // [1,2,3]
obj.powerArr(); // Object;
obj.arr; // [1, 4, 9]

var obj = {
    arr: [1,2,3],
    powerArr: function(){
        this.arr = this.arr.forEach(function(item){
            console.log(this);
            console.log(item**2);
        }, this);
    }
};