fix coupons

@@ -216,20 +216,22 @@ function pickCoupon(info) {
 }
 
 function couponSlide(num) {
-    if (!this.$btns) {
-        this.$btns = $couponBtn.children();
+    var that = this || window;
+
+    if (!that.$btns) {
+        that.$btns = $couponBtn.children();
     }
 
     $couponItem.eq(0).animate({
         'margin-left': -itemWidth * num
     });
 
-    this.$btns.removeClass('btn-end');
+    that.$btns.removeClass('btn-end');
 
     if (!num) {
-        this.$btns.eq(0).addClass('btn-end');
+        that.$btns.eq(0).addClass('btn-end');
     } else if (num >= $couponItem.length - 3) {
-        this.$btns.eq(1).addClass('btn-end');
+        that.$btns.eq(1).addClass('btn-end');
     }
 }