fix(eslint-plugin): [no-unnec-type-assert] handle JSX attributes

packages/eslint-plugin/src/rules/no-unnecessary-type-assertion.ts

@@ -1,6 +1,7 @@
 import { TSESTree } from '@typescript-eslint/experimental-utils';
 import {
   isCallExpression,
+  isJsxExpression,
   isNewExpression,
   isObjectType,
   isObjectFlagSet,
@@ -117,6 +118,8 @@ export default util.createRule<Options, MessageIds>({
         return parent.type
           ? checker.getTypeFromTypeNode(parent.type)
           : undefined;
+      } else if (isJsxExpression(parent)) {
+        return checker.getContextualType(parent);
       } else if (
         ![ts.SyntaxKind.TemplateSpan, ts.SyntaxKind.JsxExpression].includes(
           parent.kind,

packages/eslint-plugin/tests/RuleTester.ts

@@ -44,6 +44,7 @@ class RuleTester extends TSESLint.RuleTester {
   ): void {
     const errorMessage = `Do not set the parser at the test level unless you want to use a parser other than ${parser}`;
 
+    // standardize the valid tests as objects
     tests.valid = tests.valid.map(test => {
       if (typeof test === 'string') {
         return {

packages/eslint-plugin/tests/fixtures/tsconfig.json

@@ -1,10 +1,15 @@
 {
   "compilerOptions": {
+    "jsx": "preserve",
     "target": "es5",
     "module": "commonjs",
     "strict": true,
     "esModuleInterop": true,
     "lib": ["es2015", "es2017", "esnext"],
     "experimentalDecorators": true
-  }
+  },
+  "include": [
+    "file.ts",
+    "react.tsx"
+  ]
 }

packages/eslint-plugin/tests/rules/no-unnecessary-type-assertion.test.ts

@@ -117,6 +117,19 @@ function testFunction(_param: string | null): void { /* noop */ }
 const value = 'test' as string | null | undefined
 testFunction(value!)
     `,
+    // https://github.com/typescript-eslint/typescript-eslint/issues/982
+    {
+      code: `
+declare namespace JSX { interface IntrinsicElements { div: { key?: string | number } } }
+
+function Test(props: {
+  id?: null | string | number;
+}) {
+  return <div key={props.id!} />;
+}
+      `,
+      filename: path.join(rootDir, 'react.tsx'),
+    },
   ],
 
   invalid: [
@@ -327,5 +340,33 @@ class Mx {
         },
       ],
     },
+    // https://github.com/typescript-eslint/typescript-eslint/issues/982
+    {
+      code: `
+declare namespace JSX { interface IntrinsicElements { div: { key?: string | number } } }
+
+function Test(props: {
+  id?: string | number;
+}) {
+  return <div key={props.id!} />;
+}
+      `,
+      output: `
+declare namespace JSX { interface IntrinsicElements { div: { key?: string | number } } }
+
+function Test(props: {
+  id?: string | number;
+}) {
+  return <div key={props.id} />;
+}
+      `,
+      errors: [
+        {
+          messageId: 'contextuallyUnnecessary',
+          line: 7,
+        },
+      ],
+      filename: path.join(rootDir, 'react.tsx'),
+    },
   ],
 });