typescript-eslint · bradzacher · Nov 11, 2019 · Nov 4, 2019 · Nov 4, 2019 · Nov 4, 2019
diff --git a/packages/eslint-plugin/README.md b/packages/eslint-plugin/README.md
@@ -145,7 +145,7 @@ Then you should add `airbnb` (or `airbnb-base`) to your `extends` section of `.e
 | [`@typescript-eslint/await-thenable`](./docs/rules/await-thenable.md)                                     | Disallows awaiting a value that is not a Thenable                                                                                                   | :heavy_check_mark: |          | :thought_balloon: |
 | [`@typescript-eslint/ban-ts-ignore`](./docs/rules/ban-ts-ignore.md)                                       | Bans “// @ts-ignore” comments from being used                                                                                                       | :heavy_check_mark: |          |                   |
 | [`@typescript-eslint/ban-types`](./docs/rules/ban-types.md)                                               | Bans specific types from being used                                                                                                                 | :heavy_check_mark: | :wrench: |                   |
-| [`@typescript-eslint/brace-style`](./docs/rules/brace-style.md)                                               | Enforce consistent brace style for blocks                                                                                                                  |                    | :wrench: |                   |
+| [`@typescript-eslint/brace-style`](./docs/rules/brace-style.md)                                           | Enforce consistent brace style for blocks                                                                                                           |                    | :wrench: |                   |
 | [`@typescript-eslint/camelcase`](./docs/rules/camelcase.md)                                               | Enforce camelCase naming convention                                                                                                                 | :heavy_check_mark: |          |                   |
 | [`@typescript-eslint/class-name-casing`](./docs/rules/class-name-casing.md)                               | Require PascalCased class and interface names                                                                                                       | :heavy_check_mark: |          |                   |
 | [`@typescript-eslint/consistent-type-assertions`](./docs/rules/consistent-type-assertions.md)             | Enforces consistent usage of type assertions.                                                                                                       | :heavy_check_mark: |          |                   |
@@ -181,6 +181,7 @@ Then you should add `airbnb` (or `airbnb-base`) to your `extends` section of `.e
 | [`@typescript-eslint/no-unnecessary-qualifier`](./docs/rules/no-unnecessary-qualifier.md)                 | Warns when a namespace qualifier is unnecessary                                                                                                     |                    | :wrench: | :thought_balloon: |
 | [`@typescript-eslint/no-unnecessary-type-arguments`](./docs/rules/no-unnecessary-type-arguments.md)       | Warns if an explicitly specified type argument is the default for that type parameter                                                               |                    | :wrench: | :thought_balloon: |
 | [`@typescript-eslint/no-unnecessary-type-assertion`](./docs/rules/no-unnecessary-type-assertion.md)       | Warns if a type assertion does not change the type of an expression                                                                                 | :heavy_check_mark: | :wrench: | :thought_balloon: |
+| [`@typescript-eslint/no-unused-expressions`](./docs/rules/no-unused-expressions.md)                       | Disallow unused expressions                                                                                                                         |                    |          |                   |
 | [`@typescript-eslint/no-unused-vars`](./docs/rules/no-unused-vars.md)                                     | Disallow unused variables                                                                                                                           | :heavy_check_mark: |          |                   |
 | [`@typescript-eslint/no-use-before-define`](./docs/rules/no-use-before-define.md)                         | Disallow the use of variables before they are defined                                                                                               | :heavy_check_mark: |          |                   |
 | [`@typescript-eslint/no-useless-constructor`](./docs/rules/no-useless-constructor.md)                     | Disallow unnecessary constructors                                                                                                                   |                    |          |                   |
@@ -193,7 +194,7 @@ Then you should add `airbnb` (or `airbnb-base`) to your `extends` section of `.e
 | [`@typescript-eslint/prefer-regexp-exec`](./docs/rules/prefer-regexp-exec.md)                             | Prefer RegExp#exec() over String#match() if no global flag is provided                                                                              | :heavy_check_mark: |          | :thought_balloon: |
 | [`@typescript-eslint/prefer-string-starts-ends-with`](./docs/rules/prefer-string-starts-ends-with.md)     | Enforce the use of `String#startsWith` and `String#endsWith` instead of other equivalent methods of checking substrings                             | :heavy_check_mark: | :wrench: | :thought_balloon: |
 | [`@typescript-eslint/promise-function-async`](./docs/rules/promise-function-async.md)                     | Requires any function or method that returns a Promise to be marked async                                                                           |                    |          | :thought_balloon: |
-| [`@typescript-eslint/quotes`](./docs/rules/quotes.md)                                                         | Enforce the consistent use of either backticks, double, or single quotes                                                                                                       |                    | :wrench: |                   |
+| [`@typescript-eslint/quotes`](./docs/rules/quotes.md)                                                     | Enforce the consistent use of either backticks, double, or single quotes                                                                            |                    | :wrench: |                   |
 | [`@typescript-eslint/require-array-sort-compare`](./docs/rules/require-array-sort-compare.md)             | Enforce giving `compare` argument to `Array#sort`                                                                                                   |                    |          | :thought_balloon: |
 | [`@typescript-eslint/require-await`](./docs/rules/require-await.md)                                       | Disallow async functions which have no `await` expression                                                                                           | :heavy_check_mark: |          | :thought_balloon: |
 | [`@typescript-eslint/restrict-plus-operands`](./docs/rules/restrict-plus-operands.md)                     | When adding two variables, operands must both be of type number or of type string                                                                   |                    |          | :thought_balloon: |

diff --git a/packages/eslint-plugin/docs/rules/no-unused-expressions.md b/packages/eslint-plugin/docs/rules/no-unused-expressions.md
@@ -0,0 +1,25 @@
+# require or disallow semicolons instead of ASI (semi)
+
+This rule aims to eliminate unused expressions which have no effect on the state of the program.
+
+## Rule Details
+
+This rule extends the base [eslint/no-unused-expressions](https://eslint.org/docs/rules/no-unused-expressions) rule.
+It supports all options and features of the base rule.
+This version adds support for numerous typescript features.
+
+## How to use
+
+```cjson
+{
+  // note you must disable the base rule as it can report incorrect errors
+  "no-unused-expressions": "off",
+  "@typescript-eslint/no-unused-expressions": ["error"]
+}
+```
+
+## Options
+
+See [eslint/no-unused-expressions options](https://eslint.org/docs/rules/no-unused-expressions#options).
+
+<sup>Taken with ❤️ [from ESLint core](https://github.com/eslint/eslint/blob/master/docs/rules/no-unused-expressions.md)</sup>
diff --git a/packages/eslint-plugin/src/configs/all.json b/packages/eslint-plugin/src/configs/all.json
@@ -50,6 +50,7 @@
     "@typescript-eslint/no-unnecessary-qualifier": "error",
     "@typescript-eslint/no-unnecessary-type-arguments": "error",
     "@typescript-eslint/no-unnecessary-type-assertion": "error",
+    "@typescript-eslint/no-unused-expressions": "error",
     "no-unused-vars": "off",
     "@typescript-eslint/no-unused-vars": "error",
     "no-use-before-define": "off",

diff --git a/packages/eslint-plugin/src/rules/index.ts b/packages/eslint-plugin/src/rules/index.ts
@@ -39,6 +39,7 @@ import noUnnecessaryCondition from './no-unnecessary-condition';
 import noUnnecessaryQualifier from './no-unnecessary-qualifier';
 import noUnnecessaryTypeAssertion from './no-unnecessary-type-assertion';
 import noUnusedVars from './no-unused-vars';
+import noUnusedExpressions from './no-unused-expressions';
 import noUseBeforeDefine from './no-use-before-define';
 import noUselessConstructor from './no-useless-constructor';
 import noVarRequires from './no-var-requires';
@@ -106,6 +107,7 @@ export default {
   'no-unnecessary-type-arguments': useDefaultTypeParameter,
   'no-unnecessary-type-assertion': noUnnecessaryTypeAssertion,
   'no-unused-vars': noUnusedVars,
+  'no-unused-expressions': noUnusedExpressions,
   'no-use-before-define': noUseBeforeDefine,
   'no-useless-constructor': noUselessConstructor,
   'no-var-requires': noVarRequires,

diff --git a/packages/eslint-plugin/src/rules/no-unused-expressions.ts b/packages/eslint-plugin/src/rules/no-unused-expressions.ts
@@ -0,0 +1,33 @@
+import { AST_NODE_TYPES } from '@typescript-eslint/experimental-utils';
+import baseRule from 'eslint/lib/rules/no-unused-expressions';
+import * as util from '../util';
+
+export default util.createRule({
+  name: 'no-unused-expressions',
+  meta: {
+    type: 'suggestion',
+    docs: {
+      description: 'Disallow unused expressions',
+      category: 'Best Practices',
+      recommended: false,
+    },
+    schema: baseRule.meta.schema,
+    messages: {
+      expected:
+        'Expected an assignment or function call and instead saw an expression.',
+    },
+  },
+  defaultOptions: [],
+  create(context) {
+    const rules = baseRule.create(context);
+
+    return {
+      ExpressionStatement(node): void {
+        if (node.expression.type === AST_NODE_TYPES.OptionalCallExpression) {
+          return;
+        }
+        rules.ExpressionStatement(node);
+      },
+    };
+  },
+});
diff --git a/packages/eslint-plugin/tests/rules/no-unused-expressions.test.ts b/packages/eslint-plugin/tests/rules/no-unused-expressions.test.ts
@@ -0,0 +1,180 @@
+import rule from '../../src/rules/no-unused-expressions';
+import { RuleTester } from '../RuleTester';
+
+const ruleTester = new RuleTester({
+  parserOptions: {
+    ecmaVersion: 6,
+    sourceType: 'module',
+    ecmaFeatures: {},
+  },
+  parser: '@typescript-eslint/parser',
+});
+
+// the base rule doesn't have messageIds
+function error(
+  messages: { line: number; column: number }[],
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+): any[] {
+  return messages.map(message => ({
+    ...message,
+    message:
+      'Expected an assignment or function call and instead saw an expression.',
+  }));
+}
+
+ruleTester.run('no-unused-expressions', rule, {
+  valid: [
+    `
+      test.age?.toLocaleString();
+    `,
+    `
+      let a = (a?.b).c;
+    `,
+    `
+      let b = a?.['b'];
+    `,
+    `
+      let c = one[2]?.[3][4];
+    `,
+    `
+      one[2]?.[3][4]?.();
+    `,
+    `
+      a?.['b']?.c();
+    `,
+  ],
+  invalid: [
+    {
+      code: `
+if(0) 0
+        `,
+      errors: error([
+        {
+          line: 2,
+          column: 7,
+        },
+      ]),
+    },
+    {
+      code: `
+f(0), {}
+        `,
+      errors: error([
+        {
+          line: 2,
+          column: 1,
+        },
+      ]),
+    },
+    {
+      code: `
+a, b()
+        `,
+      errors: error([
+        {
+          line: 2,
+          column: 1,
+        },
+      ]),
+    },
+    {
+      code: `
+a() && function namedFunctionInExpressionContext () {f();}
+        `,
+      errors: error([
+        {
+          line: 2,
+          column: 1,
+        },
+      ]),
+    },
+    {
+      code: `
+a?.b
+        `,
+      errors: error([
+        {
+          line: 2,
+          column: 1,
+        },
+      ]),
+    },
+    {
+      code: `
+(a?.b).c
+        `,
+      errors: error([
+        {
+          line: 2,
+          column: 1,
+        },
+      ]),
+    },
+    {
+      code: `
+a?.['b']
+        `,
+      errors: error([
+        {
+          line: 2,
+          column: 1,
+        },
+      ]),
+    },
+    {
+      code: `
+(a?.['b']).c
+        `,
+      errors: error([
+        {
+          line: 2,
+          column: 1,
+        },
+      ]),
+    },
+    {
+      code: `
+a?.b()?.c
+        `,
+      errors: error([
+        {
+          line: 2,
+          column: 1,
+        },
+      ]),
+    },
+    {
+      code: `
+(a?.b()).c
+        `,
+      errors: error([
+        {
+          line: 2,
+          column: 1,
+        },
+      ]),
+    },
+    {
+      code: `
+one[2]?.[3][4];
+        `,
+      errors: error([
+        {
+          line: 2,
+          column: 1,
+        },
+      ]),
+    },
+    {
+      code: `
+one.two?.three.four;
+        `,
+      errors: error([
+        {
+          line: 2,
+          column: 1,
+        },
+      ]),
+    },
+  ],
+});
diff --git a/packages/eslint-plugin/typings/eslint-rules.d.ts b/packages/eslint-plugin/typings/eslint-rules.d.ts
@@ -304,6 +304,26 @@ declare module 'eslint/lib/rules/no-unused-vars' {
   export = rule;
 }
 
+declare module 'eslint/lib/rules/no-unused-expressions' {
+  import { TSESLint, TSESTree } from '@typescript-eslint/experimental-utils';
+
+  const rule: TSESLint.RuleModule<
+    'expected',
+    (
+      | 'all'
+      | 'local'
+      | {
+          allowShortCircuit?: boolean;
+          allowTernary?: boolean;
+          allowTaggedTemplates?: boolean;
+        })[],
+    {
+      ExpressionStatement(node: TSESTree.ExpressionStatement): void;
+    }
+  >;
+  export = rule;
+}
+
 declare module 'eslint/lib/rules/no-use-before-define' {
   import { TSESLint, TSESTree } from '@typescript-eslint/experimental-utils';