diff --git a/src/compiler/sys.ts b/src/compiler/sys.ts
index 2a4616732dd..be95e06eb46 100644
--- a/src/compiler/sys.ts
+++ b/src/compiler/sys.ts
@@ -428,7 +428,7 @@ namespace ts {
         newLine: string;
         useCaseSensitiveFileNames: boolean;
         write(s: string): void;
-        writeOutputIsTty?(): boolean;
+        writeOutputIsTTY?(): boolean;
         readFile(path: string, encoding?: string): string | undefined;
         getFileSize?(path: string): number;
         writeFile(path: string, data: string, writeByteOrderMark?: boolean): void;
@@ -562,7 +562,7 @@ namespace ts {
                 write(s: string): void {
                     process.stdout.write(s);
                 },
-                writeOutputIsTty() {
+                writeOutputIsTTY() {
                     return process.stdout.isTTY;
                 },
                 readFile,
diff --git a/src/compiler/tsc.ts b/src/compiler/tsc.ts
index b2bffd83ca7..3c73eba86ef 100644
--- a/src/compiler/tsc.ts
+++ b/src/compiler/tsc.ts
@@ -25,8 +25,8 @@ namespace ts {
     }
 
     function shouldBePretty(options: CompilerOptions) {
-        if ((typeof options.pretty === "undefined")) {
-            return !!sys.writeOutputIsTty && sys.writeOutputIsTty();
+        if (typeof options.pretty === "undefined") {
+            return !!sys.writeOutputIsTTY && sys.writeOutputIsTTY();
         }
         return options.pretty;
     }