From 990f1c7c37533e63fa828b1b816efde7411ce012 Mon Sep 17 00:00:00 2001
From: Yui <yuit@users.noreply.github.com>
Date: Thu, 5 May 2016 14:18:12 -0700
Subject: [PATCH] Check that token is AsyncKeyword before calling lookAhead
 (#8477)

* Check that token is AsyncKeyword before calling lookAhead

* Fix linting errors
---
 src/compiler/parser.ts | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/src/compiler/parser.ts b/src/compiler/parser.ts
index 8d3eacc3dfe..b4995a103f4 100644
--- a/src/compiler/parser.ts
+++ b/src/compiler/parser.ts
@@ -2989,11 +2989,14 @@ namespace ts {
         }
 
         function tryParseAsyncSimpleArrowFunctionExpression(): ArrowFunction {
-            const isUnParenthesizedAsyncArrowFunction = lookAhead(isUnParenthesizedAsyncArrowFunctionWorker);
-            if (isUnParenthesizedAsyncArrowFunction === Tristate.True) {
-                const asyncModifier = parseModifiersForArrowFunction();
-                const expr = parseBinaryExpressionOrHigher(/*precedence*/ 0);
-                return parseSimpleArrowFunctionExpression(<Identifier>expr, asyncModifier);
+            // We do a check here so that we won't be doing unnecessarily call to "lookAhead"
+            if (token === SyntaxKind.AsyncKeyword) {
+                const isUnParenthesizedAsyncArrowFunction = lookAhead(isUnParenthesizedAsyncArrowFunctionWorker);
+                if (isUnParenthesizedAsyncArrowFunction === Tristate.True) {
+                    const asyncModifier = parseModifiersForArrowFunction();
+                    const expr = parseBinaryExpressionOrHigher(/*precedence*/ 0);
+                    return parseSimpleArrowFunctionExpression(<Identifier>expr, asyncModifier);
+                }
             }
             return undefined;
         }