From d46653a2ac1f744c0abc437dc2144226346050e6 Mon Sep 17 00:00:00 2001
From: Andy <anhans@microsoft.com>
Date: Fri, 19 Jan 2018 10:10:43 -0800
Subject: [PATCH] Handle `undefined` input to firstDefined (#21300)

---
 src/compiler/checker.ts               | 2 +-
 src/compiler/core.ts                  | 4 ++++
 src/services/codefixes/importFixes.ts | 2 +-
 3 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 519a9f4cf40..859f0aa1d82 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -6562,7 +6562,7 @@ namespace ts {
             // b) It references `arguments` somewhere
             const lastParam = lastOrUndefined(declaration.parameters);
             const lastParamTags = lastParam && getJSDocParameterTags(lastParam);
-            const lastParamVariadicType = lastParamTags && firstDefined(lastParamTags, p =>
+            const lastParamVariadicType = firstDefined(lastParamTags, p =>
                 p.typeExpression && isJSDocVariadicType(p.typeExpression.type) ? p.typeExpression.type : undefined);
             if (!lastParamVariadicType && !containsArgumentsReference(declaration)) {
                 return false;
diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index afdc65de51d..cae9c841272 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -183,6 +183,10 @@ namespace ts {
 
     /** Like `forEach`, but suitable for use with numbers and strings (which may be falsy). */
     export function firstDefined<T, U>(array: ReadonlyArray<T> | undefined, callback: (element: T, index: number) => U | undefined): U | undefined {
+        if (array === undefined) {
+            return undefined;
+        }
+
         for (let i = 0; i < array.length; i++) {
             const result = callback(array[i], i);
             if (result !== undefined) {
diff --git a/src/services/codefixes/importFixes.ts b/src/services/codefixes/importFixes.ts
index 7e0aefef139..4c536ce3fd2 100644
--- a/src/services/codefixes/importFixes.ts
+++ b/src/services/codefixes/importFixes.ts
@@ -467,7 +467,7 @@ namespace ts.codefix {
         addJsExtension: boolean,
     ): string | undefined {
         const roots = getEffectiveTypeRoots(options, host);
-        return roots && firstDefined(roots, unNormalizedTypeRoot => {
+        return firstDefined(roots, unNormalizedTypeRoot => {
             const typeRoot = toPath(unNormalizedTypeRoot, /*basePath*/ undefined, getCanonicalFileName);
             if (startsWith(moduleFileName, typeRoot)) {
                 return removeExtensionAndIndexPostFix(moduleFileName.substring(typeRoot.length + 1), options, addJsExtension);