diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index 5e4aaf89aa1..362d0834cd8 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -242,9 +242,17 @@ namespace ts {
             const count = array.length;
             if (count > 0) {
                 let pos = 0;
-                let result = arguments.length <= 2 ? array[pos++] : initial;
+                let result: T | U;
+                if (arguments.length <= 2) {
+                    result = array[pos];
+                    pos++;
+                }
+                else {
+                    result = initial;
+                }
                 while (pos < count) {
-                    result = f(<U>result, array[pos++]);
+                    result = f(<U>result, array[pos]);
+                    pos++;
                 }
                 return <U>result;
             }
@@ -258,9 +266,17 @@ namespace ts {
         if (array) {
             let pos = array.length - 1;
             if (pos >= 0) {
-                let result = arguments.length <= 2 ? array[pos--] : initial;
+                let result: T | U;
+                if (arguments.length <= 2) {
+                    result = array[pos];
+                    pos--;
+                }
+                else {
+                    result = initial;
+                }
                 while (pos >= 0) {
-                    result = f(<U>result, array[pos--]);
+                    result = f(<U>result, array[pos]);
+                    pos--;
                 }
                 return <U>result;
             }