From fed89f24ebe42a6412f0eb19949d5d4771656189 Mon Sep 17 00:00:00 2001
From: Brad Zacher <brad.zacher@gmail.com>
Date: Tue, 1 Dec 2020 13:07:19 -0800
Subject: [PATCH] fix(eslint-plugin): [method-signature-style] fix crash with
 methods without a return type (#2836)

Fixes #2834
---
 .../src/rules/method-signature-style.ts       | 19 ++++++++++---------
 .../rules/method-signature-style.test.ts      | 19 +++++++++++++++++++
 2 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/packages/eslint-plugin/src/rules/method-signature-style.ts b/packages/eslint-plugin/src/rules/method-signature-style.ts
index de6f366b673..54e68b2c671 100644
--- a/packages/eslint-plugin/src/rules/method-signature-style.ts
+++ b/packages/eslint-plugin/src/rules/method-signature-style.ts
@@ -82,7 +82,11 @@ export default util.createRule<Options, MessageIds>({
     function getMethodReturnType(
       node: TSESTree.TSMethodSignature | TSESTree.TSFunctionType,
     ): string {
-      return sourceCode.getText(node.returnType!.typeAnnotation);
+      return node.returnType == null
+        ? // if the method has no return type, it implicitly has an `any` return type
+          // we just make it explicit here so we can do the fix
+          'any'
+        : sourceCode.getText(node.returnType.typeAnnotation);
     }
 
     function getDelimiter(node: TSESTree.Node): string {
@@ -149,16 +153,13 @@ export default util.createRule<Options, MessageIds>({
                   methodNode,
                   ...duplicatedKeyMethodNodes,
                 ].sort((a, b) => (a.range[0] < b.range[0] ? -1 : 1));
-                const typeString = methodNodes.reduce(
-                  (str, node, idx, nodes) => {
+                const typeString = methodNodes
+                  .map(node => {
                     const params = getMethodParams(node);
                     const returnType = getMethodReturnType(node);
-                    return `${str}(${params} => ${returnType})${
-                      idx !== nodes.length - 1 ? ' & ' : ''
-                    }`;
-                  },
-                  '',
-                );
+                    return `(${params} => ${returnType})`;
+                  })
+                  .join(' & ');
                 const key = getMethodKey(methodNode);
                 const delimiter = getDelimiter(methodNode);
                 yield fixer.replaceText(
diff --git a/packages/eslint-plugin/tests/rules/method-signature-style.test.ts b/packages/eslint-plugin/tests/rules/method-signature-style.test.ts
index e322c63c6a3..e82ee2e750b 100644
--- a/packages/eslint-plugin/tests/rules/method-signature-style.test.ts
+++ b/packages/eslint-plugin/tests/rules/method-signature-style.test.ts
@@ -427,5 +427,24 @@ declare const Foo: {
         },
       ],
     },
+    // https://github.com/typescript-eslint/typescript-eslint/issues/2834
+    {
+      code: `
+interface MyInterface {
+  methodReturningImplicitAny();
+}
+      `,
+      output: `
+interface MyInterface {
+  methodReturningImplicitAny: () => any;
+}
+      `,
+      errors: [
+        {
+          messageId: 'errorMethod',
+          line: 3,
+        },
+      ],
+    },
   ],
 });