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feat(eslint-plugin): add switch-exhaustiveness-check rule (#972)
Co-authored-by: Serg Nesterov <i.am.cust0dian@gmail.com> Co-authored-by: Brad Zacher <brad.zacher@gmail.com>
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packages/eslint-plugin/docs/rules/switch-exhaustiveness-check.md
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| # Exhaustiveness checking in switch with union type (`switch-exhaustiveness-check`) | ||
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| Union type may have a lot of parts. It's easy to forget to consider all cases in switch. This rule reminds which parts are missing. If domain of the problem requires to have only a partial switch, developer may _explicitly_ add a default clause. | ||
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| Examples of **incorrect** code for this rule: | ||
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| ```ts | ||
| type Day = | ||
| | 'Monday' | ||
| | 'Tuesday' | ||
| | 'Wednesday' | ||
| | 'Thursday' | ||
| | 'Friday' | ||
| | 'Saturday' | ||
| | 'Sunday'; | ||
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| const day = 'Monday' as Day; | ||
| let result = 0; | ||
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| switch (day) { | ||
| case 'Monday': { | ||
| result = 1; | ||
| break; | ||
| } | ||
| } | ||
| ``` | ||
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| Examples of **correct** code for this rule: | ||
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| ```ts | ||
| type Day = | ||
| | 'Monday' | ||
| | 'Tuesday' | ||
| | 'Wednesday' | ||
| | 'Thursday' | ||
| | 'Friday' | ||
| | 'Saturday' | ||
| | 'Sunday'; | ||
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| const day = 'Monday' as Day; | ||
| let result = 0; | ||
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| switch (day) { | ||
| case 'Monday': { | ||
| result = 1; | ||
| break; | ||
| } | ||
| case 'Tuesday': { | ||
| result = 2; | ||
| break; | ||
| } | ||
| case 'Wednesday': { | ||
| result = 3; | ||
| break; | ||
| } | ||
| case 'Thursday': { | ||
| result = 4; | ||
| break; | ||
| } | ||
| case 'Friday': { | ||
| result = 5; | ||
| break; | ||
| } | ||
| case 'Saturday': { | ||
| result = 6; | ||
| break; | ||
| } | ||
| case 'Sunday': { | ||
| result = 7; | ||
| break; | ||
| } | ||
| } | ||
| ``` | ||
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| or | ||
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| ```ts | ||
| type Day = | ||
| | 'Monday' | ||
| | 'Tuesday' | ||
| | 'Wednesday' | ||
| | 'Thursday' | ||
| | 'Friday' | ||
| | 'Saturday' | ||
| | 'Sunday'; | ||
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| const day = 'Monday' as Day; | ||
| let result = 0; | ||
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| switch (day) { | ||
| case 'Monday': { | ||
| result = 1; | ||
| break; | ||
| } | ||
| default: { | ||
| result = 42; | ||
| } | ||
| } | ||
| ``` | ||
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| ## When Not To Use It | ||
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| If program doesn't have union types with many parts. Downside of this rule is the need for type information, so it's slower than regular rules. |
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152
packages/eslint-plugin/src/rules/switch-exhaustiveness-check.ts
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| Original file line number | Diff line number | Diff line change |
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| import { TSESLint, TSESTree } from '@typescript-eslint/experimental-utils'; | ||
| import * as ts from 'typescript'; | ||
| import { | ||
| createRule, | ||
| getParserServices, | ||
| getConstrainedTypeAtLocation, | ||
| } from '../util'; | ||
| import { isTypeFlagSet, unionTypeParts } from 'tsutils'; | ||
| import { isClosingBraceToken, isOpeningBraceToken } from 'eslint-utils'; | ||
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| export default createRule({ | ||
| name: 'switch-exhaustiveness-check', | ||
| meta: { | ||
| type: 'suggestion', | ||
| docs: { | ||
| description: 'Exhaustiveness checking in switch with union type', | ||
| category: 'Best Practices', | ||
| recommended: false, | ||
| requiresTypeChecking: true, | ||
| }, | ||
| schema: [], | ||
| messages: { | ||
| switchIsNotExhaustive: | ||
| 'Switch is not exhaustive. Cases not matched: {{missingBranches}}', | ||
| addMissingCases: 'Add branches for missing cases', | ||
| }, | ||
| }, | ||
| defaultOptions: [], | ||
| create(context) { | ||
| const sourceCode = context.getSourceCode(); | ||
| const service = getParserServices(context); | ||
| const checker = service.program.getTypeChecker(); | ||
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| function getNodeType(node: TSESTree.Node): ts.Type { | ||
| const tsNode = service.esTreeNodeToTSNodeMap.get(node); | ||
| return getConstrainedTypeAtLocation(checker, tsNode); | ||
| } | ||
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| function fixSwitch( | ||
| fixer: TSESLint.RuleFixer, | ||
| node: TSESTree.SwitchStatement, | ||
| missingBranchTypes: Array<ts.Type>, | ||
| ): TSESLint.RuleFix | null { | ||
| const lastCase = | ||
| node.cases.length > 0 ? node.cases[node.cases.length - 1] : null; | ||
| const caseIndent = lastCase | ||
| ? ' '.repeat(lastCase.loc.start.column) | ||
| : // if there are no cases, use indentation of the switch statement | ||
| // and leave it to user to format it correctly | ||
| ' '.repeat(node.loc.start.column); | ||
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| const missingCases = []; | ||
| for (const missingBranchType of missingBranchTypes) { | ||
| // While running this rule on checker.ts of TypeScript project | ||
| // the fix introduced a compiler error due to: | ||
| // | ||
| // type __String = (string & { | ||
| // __escapedIdentifier: void; | ||
| // }) | (void & { | ||
| // __escapedIdentifier: void; | ||
| // }) | InternalSymbolName; | ||
| // | ||
| // The following check fixes it. | ||
| if (missingBranchType.isIntersection()) { | ||
| continue; | ||
| } | ||
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| const caseTest = checker.typeToString(missingBranchType); | ||
| const errorMessage = `Not implemented yet: ${caseTest} case`; | ||
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| missingCases.push( | ||
| `case ${caseTest}: { throw new Error('${errorMessage}') }`, | ||
| ); | ||
| } | ||
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| const fixString = missingCases | ||
| .map(code => `${caseIndent}${code}`) | ||
| .join('\n'); | ||
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| if (lastCase) { | ||
| return fixer.insertTextAfter(lastCase, `\n${fixString}`); | ||
| } | ||
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| // there were no existing cases | ||
| const openingBrace = sourceCode.getTokenAfter( | ||
| node.discriminant, | ||
| isOpeningBraceToken, | ||
| )!; | ||
| const closingBrace = sourceCode.getTokenAfter( | ||
| node.discriminant, | ||
| isClosingBraceToken, | ||
| )!; | ||
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| return fixer.replaceTextRange( | ||
| [openingBrace.range[0], closingBrace.range[1]], | ||
| ['{', fixString, `${caseIndent}}`].join('\n'), | ||
| ); | ||
| } | ||
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| function checkSwitchExhaustive(node: TSESTree.SwitchStatement): void { | ||
| const discriminantType = getNodeType(node.discriminant); | ||
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| if (discriminantType.isUnion()) { | ||
| const unionTypes = unionTypeParts(discriminantType); | ||
| const caseTypes: Set<ts.Type> = new Set(); | ||
| for (const switchCase of node.cases) { | ||
| if (switchCase.test === null) { | ||
| // Switch has 'default' branch - do nothing. | ||
| return; | ||
| } | ||
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| caseTypes.add(getNodeType(switchCase.test)); | ||
| } | ||
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| const missingBranchTypes = unionTypes.filter( | ||
| unionType => !caseTypes.has(unionType), | ||
| ); | ||
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| if (missingBranchTypes.length === 0) { | ||
| // All cases matched - do nothing. | ||
| return; | ||
| } | ||
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| context.report({ | ||
| node: node.discriminant, | ||
| messageId: 'switchIsNotExhaustive', | ||
| data: { | ||
| missingBranches: missingBranchTypes | ||
| .map(missingType => | ||
| isTypeFlagSet(missingType, ts.TypeFlags.ESSymbolLike) | ||
| ? `typeof ${missingType.symbol.escapedName}` | ||
| : checker.typeToString(missingType), | ||
| ) | ||
| .join(' | '), | ||
| }, | ||
| suggest: [ | ||
| { | ||
| messageId: 'addMissingCases', | ||
| fix(fixer): TSESLint.RuleFix | null { | ||
| return fixSwitch(fixer, node, missingBranchTypes); | ||
| }, | ||
| }, | ||
| ], | ||
| }); | ||
| } | ||
| } | ||
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| return { | ||
| SwitchStatement: checkSwitchExhaustive, | ||
| }; | ||
| }, | ||
| }); |
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