From 2e9c2028a8a0b226e0f87d4bcc997fa259ca3ebd Mon Sep 17 00:00:00 2001
From: Gareth Jones <Jones258@Gmail.com>
Date: Mon, 13 Apr 2020 12:50:26 +1200
Subject: [PATCH] feat(eslint-plugin): [explicit-function-return-type] add
 option to allow concise arrows that start with void (#1732)

---
 .../rules/explicit-function-return-type.md    | 20 ++++++++++
 .../rules/explicit-function-return-type.ts    | 20 +++++++++-
 .../explicit-function-return-type.test.ts     | 39 +++++++++++++++++++
 3 files changed, 78 insertions(+), 1 deletion(-)

diff --git a/packages/eslint-plugin/docs/rules/explicit-function-return-type.md b/packages/eslint-plugin/docs/rules/explicit-function-return-type.md
index 360fdce01e6..aa94a29a0ef 100644
--- a/packages/eslint-plugin/docs/rules/explicit-function-return-type.md
+++ b/packages/eslint-plugin/docs/rules/explicit-function-return-type.md
@@ -69,6 +69,8 @@ type Options = {
   allowTypedFunctionExpressions?: boolean;
   // if true, functions immediately returning another function expression will not be checked
   allowHigherOrderFunctions?: boolean;
+  // if true, concise arrow functions that start with the void keyword will not be checked
+  allowConciseArrowFunctionExpressionStartingWithVoid?: boolean;
 };
 
 const defaults = {
@@ -198,6 +200,24 @@ function fn() {
 }
 ```
 
+### `allowConciseArrowFunctionExpressionsStartingWithVoid`
+
+Examples of **incorrect** code for this rule with `{ allowConciseArrowFunctionExpressionsStartingWithVoid: true }`:
+
+```ts
+var join = (a: string, b: string) => `${a}${b}`;
+
+const log = (message: string) => {
+  console.log(message);
+};
+```
+
+Examples of **correct** code for this rule with `{ allowConciseArrowFunctionExpressionsStartingWithVoid: true }`:
+
+```ts
+var log = (message: string) => void console.log(message);
+```
+
 ## When Not To Use It
 
 If you don't wish to prevent calling code from using function return values in unexpected ways, then
diff --git a/packages/eslint-plugin/src/rules/explicit-function-return-type.ts b/packages/eslint-plugin/src/rules/explicit-function-return-type.ts
index b8ee7ff584b..f0a5978d02f 100644
--- a/packages/eslint-plugin/src/rules/explicit-function-return-type.ts
+++ b/packages/eslint-plugin/src/rules/explicit-function-return-type.ts
@@ -1,4 +1,7 @@
-import { TSESTree } from '@typescript-eslint/experimental-utils';
+import {
+  AST_NODE_TYPES,
+  TSESTree,
+} from '@typescript-eslint/experimental-utils';
 import * as util from '../util';
 import {
   checkFunctionReturnType,
@@ -11,6 +14,7 @@ type Options = [
     allowTypedFunctionExpressions?: boolean;
     allowHigherOrderFunctions?: boolean;
     allowDirectConstAssertionInArrowFunctions?: boolean;
+    allowConciseArrowFunctionExpressionsStartingWithVoid?: boolean;
   },
 ];
 type MessageIds = 'missingReturnType';
@@ -44,6 +48,9 @@ export default util.createRule<Options, MessageIds>({
           allowDirectConstAssertionInArrowFunctions: {
             type: 'boolean',
           },
+          allowConciseArrowFunctionExpressionsStartingWithVoid: {
+            type: 'boolean',
+          },
         },
         additionalProperties: false,
       },
@@ -55,6 +62,7 @@ export default util.createRule<Options, MessageIds>({
       allowTypedFunctionExpressions: true,
       allowHigherOrderFunctions: true,
       allowDirectConstAssertionInArrowFunctions: true,
+      allowConciseArrowFunctionExpressionsStartingWithVoid: false,
     },
   ],
   create(context, [options]) {
@@ -64,6 +72,16 @@ export default util.createRule<Options, MessageIds>({
       'ArrowFunctionExpression, FunctionExpression'(
         node: TSESTree.ArrowFunctionExpression | TSESTree.FunctionExpression,
       ): void {
+        if (
+          options.allowConciseArrowFunctionExpressionsStartingWithVoid &&
+          node.type === AST_NODE_TYPES.ArrowFunctionExpression &&
+          node.expression &&
+          node.body.type === AST_NODE_TYPES.UnaryExpression &&
+          node.body.operator === 'void'
+        ) {
+          return;
+        }
+
         checkFunctionExpressionReturnType(node, options, sourceCode, loc =>
           context.report({
             node,
diff --git a/packages/eslint-plugin/tests/rules/explicit-function-return-type.test.ts b/packages/eslint-plugin/tests/rules/explicit-function-return-type.test.ts
index 46ab4944185..dd3e3815676 100644
--- a/packages/eslint-plugin/tests/rules/explicit-function-return-type.test.ts
+++ b/packages/eslint-plugin/tests/rules/explicit-function-return-type.test.ts
@@ -366,6 +366,11 @@ new Foo(1, () => {});
         },
       ],
     },
+    {
+      filename: 'test.ts',
+      code: 'const log = (message: string) => void console.log(message);',
+      options: [{ allowConciseArrowFunctionExpressionsStartingWithVoid: true }],
+    },
   ],
   invalid: [
     {
@@ -1037,5 +1042,39 @@ const func = (value: number) => ({ type: 'X', value } as const);
         },
       ],
     },
+    {
+      filename: 'test.ts',
+      code: 'const log = (message: string) => void console.log(message);',
+      options: [
+        { allowConciseArrowFunctionExpressionsStartingWithVoid: false },
+      ],
+      errors: [
+        {
+          messageId: 'missingReturnType',
+          line: 1,
+          endLine: 1,
+          column: 13,
+          endColumn: 33,
+        },
+      ],
+    },
+    {
+      filename: 'test.ts',
+      code: `
+        const log = (message: string) => {
+          void console.log(message);
+        };
+      `,
+      options: [{ allowConciseArrowFunctionExpressionsStartingWithVoid: true }],
+      errors: [
+        {
+          messageId: 'missingReturnType',
+          line: 2,
+          endLine: 2,
+          column: 21,
+          endColumn: 41,
+        },
+      ],
+    },
   ],
 });