數組為：

son_window: [{ selectedCheckbox: false},
            { selectedCheckbox: false},
            { selectedCheckbox: true},
            { selectedCheckbox: true},
            { selectedCheckbox: false}],

使用for循環計算如下

    var sum;
	for (var i = 0; i < son_window.length;i++){
        if (son_window[i].selectedCheckbox == true) {                             
               sum = sum + 1;
        } 
    }

還有filter過濾器如下：（一行代碼搞定）

var sum = son_window.filter(function (x, y) { return x.selectedCheckbox == true }).length;