feat: [no-unnecessary-type-assertion] allow as const arrow functions (typescript-eslint#876)

Zzzen · JamesHenry · commit 14c6f807ec3e · 2019-08-19T15:14:57.000+04:00
diff --git a/packages/eslint-plugin/src/rules/explicit-function-return-type.ts b/packages/eslint-plugin/src/rules/explicit-function-return-type.ts
@@ -9,6 +9,7 @@ type Options = [
     allowExpressions?: boolean;
     allowTypedFunctionExpressions?: boolean;
     allowHigherOrderFunctions?: boolean;
+    allowDirectConstAssertionInArrowFunctions?: boolean;
   },
 ];
 type MessageIds = 'missingReturnType';
@@ -39,6 +40,9 @@ export default util.createRule<Options, MessageIds>({
           allowHigherOrderFunctions: {
             type: 'boolean',
           },
+          allowDirectConstAssertionInArrowFunctions: {
+            type: 'boolean',
+          },
         },
         additionalProperties: false,
       },
@@ -49,6 +53,7 @@ export default util.createRule<Options, MessageIds>({
       allowExpressions: false,
       allowTypedFunctionExpressions: true,
       allowHigherOrderFunctions: true,
+      allowDirectConstAssertionInArrowFunctions: true,
     },
   ],
   create(context, [options]) {
@@ -203,6 +208,30 @@ export default util.createRule<Options, MessageIds>({
       );
     }
 
+    /**
+     * Checks if a function belongs to:
+     * `() => ({ action: 'xxx' }) as const`
+     */
+    function returnsConstAssertionDirectly(
+      node: TSESTree.ArrowFunctionExpression,
+    ): boolean {
+      const { body } = node;
+      if (body.type === AST_NODE_TYPES.TSAsExpression) {
+        const { typeAnnotation } = body;
+        if (typeAnnotation.type === AST_NODE_TYPES.TSTypeReference) {
+          const { typeName } = typeAnnotation;
+          if (
+            typeName.type === AST_NODE_TYPES.Identifier &&
+            typeName.name === 'const'
+          ) {
+            return true;
+          }
+        }
+      }
+
+      return false;
+    }
+
     /**
      * Checks if a function declaration/expression has a return type.
      */
@@ -263,6 +292,15 @@ export default util.createRule<Options, MessageIds>({
         }
       }
 
+      // https://github.com/typescript-eslint/typescript-eslint/issues/653
+      if (
+        node.type === AST_NODE_TYPES.ArrowFunctionExpression &&
+        options.allowDirectConstAssertionInArrowFunctions &&
+        returnsConstAssertionDirectly(node)
+      ) {
+        return;
+      }
+
       checkFunctionReturnType(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
@@ -308,6 +308,20 @@ foo({
         },
       ],
     },
+    {
+      filename: 'test.ts',
+      code: `
+const func = (value: number) => (({ type: "X", value }) as const);
+const func = (value: number) => ({ type: "X", value } as const);
+const func = (value: number) => (x as const);
+const func = (value: number) => x as const;
+      `,
+      options: [
+        {
+          allowDirectConstAssertionInArrowFunctions: true,
+        },
+      ],
+    },
   ],
   invalid: [
     {
@@ -749,5 +763,47 @@ foo({
         },
       ],
     },
+    {
+      filename: 'test.ts',
+      code: `
+const func = (value: number) => ({ type: "X", value } as any);
+const func = (value: number) => ({ type: "X", value } as Action);
+      `,
+      options: [
+        {
+          allowDirectConstAssertionInArrowFunctions: true,
+        },
+      ],
+      errors: [
+        {
+          messageId: 'missingReturnType',
+          line: 2,
+          column: 14,
+        },
+        {
+          messageId: 'missingReturnType',
+          line: 3,
+          column: 14,
+        },
+      ],
+    },
+    {
+      filename: 'test.ts',
+      code: `
+const func = (value: number) => ({ type: "X", value } as const);
+      `,
+      options: [
+        {
+          allowDirectConstAssertionInArrowFunctions: false,
+        },
+      ],
+      errors: [
+        {
+          messageId: 'missingReturnType',
+          line: 2,
+          column: 14,
+        },
+      ],
+    },
   ],
 });
