循环语句求两数间整数和运行结果一直是0

// index.js
var start, end, sum;
Page({
startNUM: function (e) {
start = parseInt(e.detail.value);
},
endNum: function (e) {
end = parseInt(e.detail.value);
},
calc: function () {
sum = 0;
for (var i = start; i <= end; i++) {
sum = sum + i;
}
this.setData({
sum: sum
})
}
})