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feat(eslint-plugin): add rule no-void-expression (typescript-eslint#2605)
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‎.eslintrc.js

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@@ -221,6 +221,15 @@ module.exports = {
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'@typescript-eslint/internal/plugin-test-formatting': 'error',
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},
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},
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// files which list all the things
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{
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files: ['packages/eslint-plugin/src/rules/index.ts'],
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rules: {
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// enforce alphabetical ordering
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'sort-keys': 'error',
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'import/order': ['error', { alphabetize: { order: 'asc' } }],
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},
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},
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// tools and tests
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{
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files: ['**/tools/**/*.ts', '**/tests/**/*.ts'],

‎packages/eslint-plugin/README.md

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@@ -117,6 +117,7 @@ Pro Tip: For larger codebases you may want to consider splitting our linting int
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| [`@typescript-eslint/naming-convention`](./docs/rules/naming-convention.md) | Enforces naming conventions for everything across a codebase | | | :thought_balloon: |
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| [`@typescript-eslint/no-base-to-string`](./docs/rules/no-base-to-string.md) | Requires that `.toString()` is only called on objects which provide useful information when stringified | | | :thought_balloon: |
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| [`@typescript-eslint/no-confusing-non-null-assertion`](./docs/rules/no-confusing-non-null-assertion.md) | Disallow non-null assertion in locations that may be confusing | | :wrench: | |
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| [`@typescript-eslint/no-confusing-void-expression`](./docs/rules/no-confusing-void-expression.md) | Requires expressions of type void to appear in statement position | | :wrench: | :thought_balloon: |
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| [`@typescript-eslint/no-dynamic-delete`](./docs/rules/no-dynamic-delete.md) | Disallow the delete operator with computed key expressions | | :wrench: | |
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| [`@typescript-eslint/no-empty-interface`](./docs/rules/no-empty-interface.md) | Disallow the declaration of empty interfaces | :heavy_check_mark: | :wrench: | |
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| [`@typescript-eslint/no-explicit-any`](./docs/rules/no-explicit-any.md) | Disallow usage of the `any` type | :heavy_check_mark: | :wrench: | |

‎packages/eslint-plugin/ROADMAP.md

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@@ -96,7 +96,7 @@ It lists all TSLint rules along side rules from the ESLint ecosystem that are th
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| [`no-unused-variable`] | 🌓 | [`@typescript-eslint/no-unused-vars`] |
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| [`no-use-before-declare`] | ✅ | [`@typescript-eslint/no-use-before-define`] |
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| [`no-var-keyword`] | 🌟 | [`no-var`][no-var] |
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| [`no-void-expression`] | 🛑 | N/A (unrelated to the similarly named ESLint rule `no-void`) |
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| [`no-void-expression`] | ✅ | [`@typescript-eslint/no-confusing-void-expression`] |
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| [`prefer-conditional-expression`] | 🛑 | N/A |
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| [`prefer-object-spread`] | 🌟 | [`prefer-object-spread`][prefer-object-spread] |
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| [`radix`] | 🌟 | [`radix`][radix] |
@@ -650,6 +650,8 @@ Relevant plugins: [`chai-expect-keywords`](https://github.com/gavinaiken/eslint-
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[`@typescript-eslint/no-floating-promises`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-floating-promises.md
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[`@typescript-eslint/no-magic-numbers`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-magic-numbers.md
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[`@typescript-eslint/no-unsafe-member-access`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-unsafe-member-access.md
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[`@typescript-eslint/restrict-template-expressions`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/restrict-template-expressions.md
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[`@typescript-eslint/no-confusing-void-expression`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-confusing-void-expression.md
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<!-- eslint-plugin-import -->
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‎packages/eslint-plugin/docs/rules/no-confusing-void-expression.md

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# Requires expressions of type void to appear in statement position (`no-confusing-void-expression`)
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Returning the results of an expression whose type is void can be misleading.
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Attempting to do so is likely a symptom of expecting a different return type from a function.
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Even if used correctly, it can be misleading for other developers,
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who don't know what a particular function does and if its result matters.
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This rule provides automatic fixes for most common cases.
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## Examples
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Examples of **incorrect** code for this rule:
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```ts
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// somebody forgot that `alert` doesn't return anything
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const response = alert('Are you sure?');
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console.log(alert('Are you sure?'));
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// it's not obvious whether the chained promise will contain the response (fixable)
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promise.then(value => window.postMessage(value));
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// it looks like we are returning the result of `console.error` (fixable)
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function doSomething() {
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if (!somethingToDo) {
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return console.error('Nothing to do!');
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}
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console.log('Doing a thing...');
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}
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```
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Examples of **correct** code for this rule:
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```ts
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// just a regular void function in a statement position
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alert('Hello, world!');
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// this function returns a boolean value so it's ok
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const response = confirm('Are you sure?');
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console.log(confirm('Are you sure?'));
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// now it's obvious that `postMessage` doesn't return any response
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promise.then(value => {
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window.postMessage(value);
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});
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// now it's explicit that we want to log the error and return early
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function doSomething() {
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if (!somethingToDo) {
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console.error('Nothing to do!');
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return;
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}
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console.log('Doing a thing...');
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}
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// using logical expressions for their side effects is fine
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cond && console.log('true');
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cond || console.error('false');
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cond ? console.log('true') : console.error('false');
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```
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## Options
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An object option can be specified. Each boolean flag makes the rule less strict.
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```ts
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type Options = {
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ignoreArrowShorthand?: boolean;
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ignoreVoidOperator?: boolean;
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};
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const defaults: Options = {
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ignoreArrowShorthand: false,
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ignoreVoidOperator: false,
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};
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```
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### `ignoreArrowShorthand`
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`false` by default.
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```json
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{
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"@typescript-eslint/no-confusing-void-expression": [
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"error",
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{ "ignoreArrowShorthand": true }
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]
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}
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```
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It might be undesirable to wrap every arrow function shorthand expression with braces.
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Especially when using Prettier formatter, which spreads such code across 3 lines instead of 1.
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Examples of additional **correct** code with this option enabled:
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```ts
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promise.then(value => window.postMessage(value));
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```
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### `ignoreVoidOperator`
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`false` by default.
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```json
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{
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"@typescript-eslint/no-confusing-void-expression": [
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"error",
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{ "ignoreVoidOperator": true }
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]
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}
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```
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It might be preferable to only use some distinct syntax
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to explicitly mark the confusing but valid usage of void expressions.
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This option allows void expressions which are explicitly wrapped in the `void` operator.
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This can help avoid confusion among other developers as long as they are made aware of this code style.
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This option also changes the automatic fixes for common cases to use the `void` operator.
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It also enables a suggestion fix to wrap the void expression with `void` operator for every problem reported.
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Examples of additional **correct** code with this option enabled:
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```ts
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// now it's obvious that we don't expect any response
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promise.then(value => void window.postMessage(value));
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// now it's explicit that we don't want to return anything
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function doSomething() {
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if (!somethingToDo) {
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return void console.error('Nothing to do!');
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}
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console.log('Doing a thing...');
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}
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// we are sure that we want to always log `undefined`
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console.log(void alert('Hello, world!'));
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```
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## When Not To Use It
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The return type of a function can be inspected by going to its definition or hovering over it in an IDE.
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If you don't care about being explicit about the void type in actual code then don't use this rule.
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Also, if you prefer concise coding style then also don't use it.
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## Related to
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- TSLint: ['no-void-expression'](https://palantir.github.io/tslint/rules/no-void-expression/)

‎packages/eslint-plugin/src/configs/all.ts

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'@typescript-eslint/no-array-constructor': 'error',
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'@typescript-eslint/no-base-to-string': 'error',
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'@typescript-eslint/no-confusing-non-null-assertion': 'error',
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'@typescript-eslint/no-confusing-void-expression': 'error',
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'no-dupe-class-members': 'off',
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'@typescript-eslint/no-dupe-class-members': 'error',
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'no-duplicate-imports': 'off',

‎packages/eslint-plugin/src/rules/index.ts

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@@ -8,14 +8,12 @@ import braceStyle from './brace-style';
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import classLiteralPropertyStyle from './class-literal-property-style';
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import commaDangle from './comma-dangle';
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import commaSpacing from './comma-spacing';
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import confusingNonNullAssertionLikeNotEqual from './no-confusing-non-null-assertion';
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import consistentIndexedObjectStyle from './consistent-indexed-object-style';
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import consistentTypeAssertions from './consistent-type-assertions';
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import consistentTypeDefinitions from './consistent-type-definitions';
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import consistentTypeImports from './consistent-type-imports';
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import defaultParamLast from './default-param-last';
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import dotNotation from './dot-notation';
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import enumMembersSpacing from './space-infix-ops';
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import explicitFunctionReturnType from './explicit-function-return-type';
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import explicitMemberAccessibility from './explicit-member-accessibility';
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import explicitModuleBoundaryTypes from './explicit-module-boundary-types';
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import namingConvention from './naming-convention';
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import noArrayConstructor from './no-array-constructor';
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import noBaseToString from './no-base-to-string';
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import confusingNonNullAssertionLikeNotEqual from './no-confusing-non-null-assertion';
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import noConfusingVoidExpression from './no-confusing-void-expression';
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import noDupeClassMembers from './no-dupe-class-members';
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import noDuplicateImports from './no-duplicate-imports';
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import noDynamicDelete from './no-dynamic-delete';
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import noEmptyFunction from './no-empty-function';
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import noEmptyInterface from './no-empty-interface';
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import noExplicitAny from './no-explicit-any';
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import noImplicitAnyCatch from './no-implicit-any-catch';
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import noExtraneousClass from './no-extraneous-class';
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import noExtraNonNullAssertion from './no-extra-non-null-assertion';
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import noExtraParens from './no-extra-parens';
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import noExtraSemi from './no-extra-semi';
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import noExtraneousClass from './no-extraneous-class';
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import noFloatingPromises from './no-floating-promises';
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import noForInArray from './no-for-in-array';
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import preferLiteralEnumMember from './prefer-literal-enum-member';
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import noImplicitAnyCatch from './no-implicit-any-catch';
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import noImpliedEval from './no-implied-eval';
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import noInferrableTypes from './no-inferrable-types';
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import noInvalidThis from './no-invalid-this';
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import noInvalidVoidType from './no-invalid-void-type';
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import noLossOfPrecision from './no-loss-of-precision';
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import noLoopFunc from './no-loop-func';
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import noLossOfPrecision from './no-loss-of-precision';
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import noMagicNumbers from './no-magic-numbers';
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import noMisusedNew from './no-misused-new';
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import noMisusedPromises from './no-misused-promises';
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import preferForOf from './prefer-for-of';
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import preferFunctionType from './prefer-function-type';
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import preferIncludes from './prefer-includes';
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import preferLiteralEnumMember from './prefer-literal-enum-member';
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import preferNamespaceKeyword from './prefer-namespace-keyword';
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import preferNullishCoalescing from './prefer-nullish-coalescing';
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import preferOptionalChain from './prefer-optional-chain';
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import returnAwait from './return-await';
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import semi from './semi';
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import spaceBeforeFunctionParen from './space-before-function-paren';
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import spaceInfixOps from './space-infix-ops';
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import strictBooleanExpressions from './strict-boolean-expressions';
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import switchExhaustivenessCheck from './switch-exhaustiveness-check';
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import tripleSlashReference from './triple-slash-reference';
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import typeAnnotationSpacing from './type-annotation-spacing';
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import typedef from './typedef';
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import unboundMethod from './unbound-method';
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import unifiedSignatures from './unified-signatures';
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import noDuplicateImports from './no-duplicate-imports';
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export default {
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'adjacent-overload-signatures': adjacentOverloadSignatures,
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'consistent-type-imports': consistentTypeImports,
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'default-param-last': defaultParamLast,
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'dot-notation': dotNotation,
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'space-infix-ops': enumMembersSpacing,
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'explicit-function-return-type': explicitFunctionReturnType,
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'explicit-member-accessibility': explicitMemberAccessibility,
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'explicit-module-boundary-types': explicitModuleBoundaryTypes,
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'func-call-spacing': funcCallSpacing,
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indent: indent,
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'init-declarations': initDeclarations,
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'keyword-spacing': keywordSpacing,
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'lines-between-class-members': linesBetweenClassMembers,
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'no-array-constructor': noArrayConstructor,
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'no-base-to-string': noBaseToString,
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'no-confusing-non-null-assertion': confusingNonNullAssertionLikeNotEqual,
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'no-confusing-void-expression': noConfusingVoidExpression,
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'no-dupe-class-members': noDupeClassMembers,
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'no-duplicate-imports': noDuplicateImports,
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'no-dynamic-delete': noDynamicDelete,
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'no-empty-function': noEmptyFunction,
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'no-empty-interface': noEmptyInterface,
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'no-unsafe-member-access': noUnsafeMemberAccess,
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'no-unsafe-return': noUnsafeReturn,
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'no-unused-expressions': noUnusedExpressions,
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'no-unused-vars-experimental': noUnusedVarsExperimental,
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'no-unused-vars': noUnusedVars,
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'no-unused-vars-experimental': noUnusedVarsExperimental,
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'no-use-before-define': noUseBeforeDefine,
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'no-useless-constructor': noUselessConstructor,
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'no-var-requires': noVarRequires,
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'prefer-namespace-keyword': preferNamespaceKeyword,
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'prefer-nullish-coalescing': preferNullishCoalescing,
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'prefer-optional-chain': preferOptionalChain,
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'prefer-readonly-parameter-types': preferReadonlyParameterTypes,
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'prefer-readonly': preferReadonly,
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'prefer-readonly-parameter-types': preferReadonlyParameterTypes,
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'prefer-reduce-type-parameter': preferReduceTypeParameter,
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'prefer-regexp-exec': preferRegexpExec,
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'prefer-string-starts-ends-with': preferStringStartsEndsWith,
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'prefer-ts-expect-error': preferTsExpectError,
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'promise-function-async': promiseFunctionAsync,
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quotes: quotes,
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'require-array-sort-compare': requireArraySortCompare,
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'require-await': requireAwait,
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'restrict-plus-operands': restrictPlusOperands,
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'restrict-template-expressions': restrictTemplateExpressions,
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'return-await': returnAwait,
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semi: semi,
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'space-before-function-paren': spaceBeforeFunctionParen,
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'space-infix-ops': spaceInfixOps,
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'strict-boolean-expressions': strictBooleanExpressions,
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'switch-exhaustiveness-check': switchExhaustivenessCheck,
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'triple-slash-reference': tripleSlashReference,
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'type-annotation-spacing': typeAnnotationSpacing,
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typedef: typedef,
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'unbound-method': unboundMethod,
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'unified-signatures': unifiedSignatures,
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'no-duplicate-imports': noDuplicateImports,
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indent: indent,
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quotes: quotes,
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semi: semi,
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typedef: typedef,
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};

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