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feat(eslint-plugin): add rule `no-void-expression` (#2605)

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phaux committed Nov 2, 2020
1 parent fb1d9b1 commit c8a4dadeab8a64fb4768deda8f65475435dd2cad
@@ -221,6 +221,15 @@ module.exports = {
'@typescript-eslint/internal/plugin-test-formatting': 'error',
// files which list all the things
files: ['packages/eslint-plugin/src/rules/index.ts'],
rules: {
// enforce alphabetical ordering
'sort-keys': 'error',
'import/order': ['error', { alphabetize: { order: 'asc' } }],
// tools and tests
files: ['**/tools/**/*.ts', '**/tests/**/*.ts'],
@@ -117,6 +117,7 @@ Pro Tip: For larger codebases you may want to consider splitting our linting int
| [`@typescript-eslint/naming-convention`](./docs/rules/ | Enforces naming conventions for everything across a codebase | | | :thought_balloon: |
| [`@typescript-eslint/no-base-to-string`](./docs/rules/ | Requires that `.toString()` is only called on objects which provide useful information when stringified | | | :thought_balloon: |
| [`@typescript-eslint/no-confusing-non-null-assertion`](./docs/rules/ | Disallow non-null assertion in locations that may be confusing | | :wrench: | |
| [`@typescript-eslint/no-confusing-void-expression`](./docs/rules/ | Requires expressions of type void to appear in statement position | | :wrench: | :thought_balloon: |
| [`@typescript-eslint/no-dynamic-delete`](./docs/rules/ | Disallow the delete operator with computed key expressions | | :wrench: | |
| [`@typescript-eslint/no-empty-interface`](./docs/rules/ | Disallow the declaration of empty interfaces | :heavy_check_mark: | :wrench: | |
| [`@typescript-eslint/no-explicit-any`](./docs/rules/ | Disallow usage of the `any` type | :heavy_check_mark: | :wrench: | |
@@ -96,7 +96,7 @@ It lists all TSLint rules along side rules from the ESLint ecosystem that are th
| [`no-unused-variable`] | 🌓 | [`@typescript-eslint/no-unused-vars`] |
| [`no-use-before-declare`] | ✅ | [`@typescript-eslint/no-use-before-define`] |
| [`no-var-keyword`] | 🌟 | [`no-var`][no-var] |
| [`no-void-expression`] | 🛑 | N/A (unrelated to the similarly named ESLint rule `no-void`) |
| [`no-void-expression`] | | [`@typescript-eslint/no-confusing-void-expression`] |
| [`prefer-conditional-expression`] | 🛑 | N/A |
| [`prefer-object-spread`] | 🌟 | [`prefer-object-spread`][prefer-object-spread] |
| [`radix`] | 🌟 | [`radix`][radix] |
@@ -650,6 +650,8 @@ Relevant plugins: [`chai-expect-keywords`](

<!-- eslint-plugin-import -->

@@ -0,0 +1,149 @@
# Requires expressions of type void to appear in statement position (`no-confusing-void-expression`)

Returning the results of an expression whose type is void can be misleading.
Attempting to do so is likely a symptom of expecting a different return type from a function.
Even if used correctly, it can be misleading for other developers,
who don't know what a particular function does and if its result matters.

This rule provides automatic fixes for most common cases.

## Examples

Examples of **incorrect** code for this rule:

// somebody forgot that `alert` doesn't return anything
const response = alert('Are you sure?');
console.log(alert('Are you sure?'));
// it's not obvious whether the chained promise will contain the response (fixable)
promise.then(value => window.postMessage(value));
// it looks like we are returning the result of `console.error` (fixable)
function doSomething() {
if (!somethingToDo) {
return console.error('Nothing to do!');
console.log('Doing a thing...');

Examples of **correct** code for this rule:

// just a regular void function in a statement position
alert('Hello, world!');
// this function returns a boolean value so it's ok
const response = confirm('Are you sure?');
console.log(confirm('Are you sure?'));
// now it's obvious that `postMessage` doesn't return any response
promise.then(value => {
// now it's explicit that we want to log the error and return early
function doSomething() {
if (!somethingToDo) {
console.error('Nothing to do!');
console.log('Doing a thing...');
// using logical expressions for their side effects is fine
cond && console.log('true');
cond || console.error('false');
cond ? console.log('true') : console.error('false');

## Options

An object option can be specified. Each boolean flag makes the rule less strict.

type Options = {
ignoreArrowShorthand?: boolean;
ignoreVoidOperator?: boolean;
const defaults: Options = {
ignoreArrowShorthand: false,
ignoreVoidOperator: false,

### `ignoreArrowShorthand`

`false` by default.

"@typescript-eslint/no-confusing-void-expression": [
{ "ignoreArrowShorthand": true }

It might be undesirable to wrap every arrow function shorthand expression with braces.
Especially when using Prettier formatter, which spreads such code across 3 lines instead of 1.

Examples of additional **correct** code with this option enabled:

promise.then(value => window.postMessage(value));

### `ignoreVoidOperator`

`false` by default.

"@typescript-eslint/no-confusing-void-expression": [
{ "ignoreVoidOperator": true }

It might be preferable to only use some distinct syntax
to explicitly mark the confusing but valid usage of void expressions.
This option allows void expressions which are explicitly wrapped in the `void` operator.
This can help avoid confusion among other developers as long as they are made aware of this code style.

This option also changes the automatic fixes for common cases to use the `void` operator.
It also enables a suggestion fix to wrap the void expression with `void` operator for every problem reported.

Examples of additional **correct** code with this option enabled:

// now it's obvious that we don't expect any response
promise.then(value => void window.postMessage(value));
// now it's explicit that we don't want to return anything
function doSomething() {
if (!somethingToDo) {
return void console.error('Nothing to do!');
console.log('Doing a thing...');
// we are sure that we want to always log `undefined`
console.log(void alert('Hello, world!'));

## When Not To Use It

The return type of a function can be inspected by going to its definition or hovering over it in an IDE.
If you don't care about being explicit about the void type in actual code then don't use this rule.
Also, if you prefer concise coding style then also don't use it.

## Related to

- TSLint: ['no-void-expression'](
@@ -47,6 +47,7 @@ export = {
'@typescript-eslint/no-array-constructor': 'error',
'@typescript-eslint/no-base-to-string': 'error',
'@typescript-eslint/no-confusing-non-null-assertion': 'error',
'@typescript-eslint/no-confusing-void-expression': 'error',
'no-dupe-class-members': 'off',
'@typescript-eslint/no-dupe-class-members': 'error',
'no-duplicate-imports': 'off',
@@ -8,14 +8,12 @@ import braceStyle from './brace-style';
import classLiteralPropertyStyle from './class-literal-property-style';
import commaDangle from './comma-dangle';
import commaSpacing from './comma-spacing';
import confusingNonNullAssertionLikeNotEqual from './no-confusing-non-null-assertion';
import consistentIndexedObjectStyle from './consistent-indexed-object-style';
import consistentTypeAssertions from './consistent-type-assertions';
import consistentTypeDefinitions from './consistent-type-definitions';
import consistentTypeImports from './consistent-type-imports';
import defaultParamLast from './default-param-last';
import dotNotation from './dot-notation';
import enumMembersSpacing from './space-infix-ops';
import explicitFunctionReturnType from './explicit-function-return-type';
import explicitMemberAccessibility from './explicit-member-accessibility';
import explicitModuleBoundaryTypes from './explicit-module-boundary-types';
@@ -30,25 +28,27 @@ import methodSignatureStyle from './method-signature-style';
import namingConvention from './naming-convention';
import noArrayConstructor from './no-array-constructor';
import noBaseToString from './no-base-to-string';
import confusingNonNullAssertionLikeNotEqual from './no-confusing-non-null-assertion';
import noConfusingVoidExpression from './no-confusing-void-expression';
import noDupeClassMembers from './no-dupe-class-members';
import noDuplicateImports from './no-duplicate-imports';
import noDynamicDelete from './no-dynamic-delete';
import noEmptyFunction from './no-empty-function';
import noEmptyInterface from './no-empty-interface';
import noExplicitAny from './no-explicit-any';
import noImplicitAnyCatch from './no-implicit-any-catch';
import noExtraneousClass from './no-extraneous-class';
import noExtraNonNullAssertion from './no-extra-non-null-assertion';
import noExtraParens from './no-extra-parens';
import noExtraSemi from './no-extra-semi';
import noExtraneousClass from './no-extraneous-class';
import noFloatingPromises from './no-floating-promises';
import noForInArray from './no-for-in-array';
import preferLiteralEnumMember from './prefer-literal-enum-member';
import noImplicitAnyCatch from './no-implicit-any-catch';
import noImpliedEval from './no-implied-eval';
import noInferrableTypes from './no-inferrable-types';
import noInvalidThis from './no-invalid-this';
import noInvalidVoidType from './no-invalid-void-type';
import noLossOfPrecision from './no-loss-of-precision';
import noLoopFunc from './no-loop-func';
import noLossOfPrecision from './no-loss-of-precision';
import noMagicNumbers from './no-magic-numbers';
import noMisusedNew from './no-misused-new';
import noMisusedPromises from './no-misused-promises';
@@ -83,6 +83,7 @@ import preferEnumInitializers from './prefer-enum-initializers';
import preferForOf from './prefer-for-of';
import preferFunctionType from './prefer-function-type';
import preferIncludes from './prefer-includes';
import preferLiteralEnumMember from './prefer-literal-enum-member';
import preferNamespaceKeyword from './prefer-namespace-keyword';
import preferNullishCoalescing from './prefer-nullish-coalescing';
import preferOptionalChain from './prefer-optional-chain';
@@ -101,14 +102,14 @@ import restrictTemplateExpressions from './restrict-template-expressions';
import returnAwait from './return-await';
import semi from './semi';
import spaceBeforeFunctionParen from './space-before-function-paren';
import spaceInfixOps from './space-infix-ops';
import strictBooleanExpressions from './strict-boolean-expressions';
import switchExhaustivenessCheck from './switch-exhaustiveness-check';
import tripleSlashReference from './triple-slash-reference';
import typeAnnotationSpacing from './type-annotation-spacing';
import typedef from './typedef';
import unboundMethod from './unbound-method';
import unifiedSignatures from './unified-signatures';
import noDuplicateImports from './no-duplicate-imports';

export default {
'adjacent-overload-signatures': adjacentOverloadSignatures,
@@ -127,11 +128,11 @@ export default {
'consistent-type-imports': consistentTypeImports,
'default-param-last': defaultParamLast,
'dot-notation': dotNotation,
'space-infix-ops': enumMembersSpacing,
'explicit-function-return-type': explicitFunctionReturnType,
'explicit-member-accessibility': explicitMemberAccessibility,
'explicit-module-boundary-types': explicitModuleBoundaryTypes,
'func-call-spacing': funcCallSpacing,
indent: indent,
'init-declarations': initDeclarations,
'keyword-spacing': keywordSpacing,
'lines-between-class-members': linesBetweenClassMembers,
@@ -142,7 +143,9 @@ export default {
'no-array-constructor': noArrayConstructor,
'no-base-to-string': noBaseToString,
'no-confusing-non-null-assertion': confusingNonNullAssertionLikeNotEqual,
'no-confusing-void-expression': noConfusingVoidExpression,
'no-dupe-class-members': noDupeClassMembers,
'no-duplicate-imports': noDuplicateImports,
'no-dynamic-delete': noDynamicDelete,
'no-empty-function': noEmptyFunction,
'no-empty-interface': noEmptyInterface,
@@ -184,8 +187,8 @@ export default {
'no-unsafe-member-access': noUnsafeMemberAccess,
'no-unsafe-return': noUnsafeReturn,
'no-unused-expressions': noUnusedExpressions,
'no-unused-vars-experimental': noUnusedVarsExperimental,
'no-unused-vars': noUnusedVars,
'no-unused-vars-experimental': noUnusedVarsExperimental,
'no-use-before-define': noUseBeforeDefine,
'no-useless-constructor': noUselessConstructor,
'no-var-requires': noVarRequires,
@@ -198,28 +201,27 @@ export default {
'prefer-namespace-keyword': preferNamespaceKeyword,
'prefer-nullish-coalescing': preferNullishCoalescing,
'prefer-optional-chain': preferOptionalChain,
'prefer-readonly-parameter-types': preferReadonlyParameterTypes,
'prefer-readonly': preferReadonly,
'prefer-readonly-parameter-types': preferReadonlyParameterTypes,
'prefer-reduce-type-parameter': preferReduceTypeParameter,
'prefer-regexp-exec': preferRegexpExec,
'prefer-string-starts-ends-with': preferStringStartsEndsWith,
'prefer-ts-expect-error': preferTsExpectError,
'promise-function-async': promiseFunctionAsync,
quotes: quotes,
'require-array-sort-compare': requireArraySortCompare,
'require-await': requireAwait,
'restrict-plus-operands': restrictPlusOperands,
'restrict-template-expressions': restrictTemplateExpressions,
'return-await': returnAwait,
semi: semi,
'space-before-function-paren': spaceBeforeFunctionParen,
'space-infix-ops': spaceInfixOps,
'strict-boolean-expressions': strictBooleanExpressions,
'switch-exhaustiveness-check': switchExhaustivenessCheck,
'triple-slash-reference': tripleSlashReference,
'type-annotation-spacing': typeAnnotationSpacing,
typedef: typedef,
'unbound-method': unboundMethod,
'unified-signatures': unifiedSignatures,
'no-duplicate-imports': noDuplicateImports,
indent: indent,
quotes: quotes,
semi: semi,
typedef: typedef,

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