diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 4c3a98b12dc..a0675e957dd 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -4260,13 +4260,11 @@ namespace ts {
 
                 if (isPropertyAccessExpression(expression.left) && expression.left.expression.kind === SyntaxKind.ThisKeyword) {
                     const thisContainer = getThisContainer(expression, /*includeArrowFunctions*/ false);
-                    const isPrototypeProperty = isBinaryExpression(thisContainer.parent) &&
-                        getSpecialPropertyAssignmentKind(thisContainer.parent) === SpecialPropertyAssignmentKind.PrototypeProperty;
                     // Properties defined in a constructor (or javascript constructor function) don't get undefined added.
                     // Function expressions that are assigned to the prototype count as methods.
                     if (thisContainer.kind === SyntaxKind.Constructor ||
                         thisContainer.kind === SyntaxKind.FunctionDeclaration ||
-                        (thisContainer.kind === SyntaxKind.FunctionExpression && !isPrototypeProperty)) {
+                        (thisContainer.kind === SyntaxKind.FunctionExpression && !isPrototypePropertyAssignment(thisContainer.parent))) {
                         definedInConstructor = true;
                     }
                     else {
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index a5ba7c30cd2..69249952166 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -1656,6 +1656,10 @@ namespace ts {
         return SpecialPropertyAssignmentKind.None;
     }
 
+    export function isPrototypePropertyAssignment(node: Node): boolean {
+        return isBinaryExpression(node) && getSpecialPropertyAssignmentKind(node) === SpecialPropertyAssignmentKind.PrototypeProperty;
+    }
+
     export function isSpecialPropertyDeclaration(expr: PropertyAccessExpression): boolean {
         return isInJavaScriptFile(expr) &&
             expr.parent && expr.parent.kind === SyntaxKind.ExpressionStatement &&