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feat(eslint-plugin): add rule strict-boolean-expressions (#579)
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packages/eslint-plugin/docs/rules/strict-boolean-expressions.md
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# Boolean expressions are limited to booleans (strict-boolean-expressions) | ||
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Requires that any boolean expression is limited to true booleans rather than | ||
casting another primitive to a boolean at runtime. | ||
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It is useful to be explicit, for example, if you were trying to check if a | ||
number was defined. Doing `if (number)` would evaluate to `false` if `number` | ||
was defined and `0`. This rule forces these expressions to be explicit and to | ||
strictly use booleans. | ||
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The following nodes are checked: | ||
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- Arguments to the `!`, `&&`, and `||` operators | ||
- The condition in a conditional expression `(cond ? x : y)` | ||
- Conditions for `if`, `for`, `while`, and `do-while` statements. | ||
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Examples of **incorrect** code for this rule: | ||
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```ts | ||
const number = 0; | ||
if (number) { | ||
return; | ||
} | ||
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let foo = bar || 'foobar'; | ||
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let undefinedItem; | ||
let foo = undefinedItem ? 'foo' : 'bar'; | ||
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let str = 'foo'; | ||
while (str) { | ||
break; | ||
} | ||
``` | ||
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Examples of **correct** code for this rule: | ||
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```ts | ||
const number = 0; | ||
if (typeof number !== 'undefined') { | ||
return; | ||
} | ||
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let foo = typeof bar !== 'undefined' ? bar : 'foobar'; | ||
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let undefinedItem; | ||
let foo = typeof undefinedItem !== 'undefined' ? 'foo' : 'bar'; | ||
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let str = 'foo'; | ||
while (typeof str !== 'undefined') { | ||
break; | ||
} | ||
``` | ||
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## Related To | ||
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- TSLint: [strict-boolean-expressions](https://palantir.github.io/tslint/rules/strict-boolean-expressions) |
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packages/eslint-plugin/src/rules/strict-boolean-expressions.ts
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import { | ||
TSESTree, | ||
AST_NODE_TYPES, | ||
} from '@typescript-eslint/experimental-utils'; | ||
import ts from 'typescript'; | ||
import * as tsutils from 'tsutils'; | ||
import * as util from '../util'; | ||
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type ExpressionWithTest = | ||
| TSESTree.ConditionalExpression | ||
| TSESTree.DoWhileStatement | ||
| TSESTree.ForStatement | ||
| TSESTree.IfStatement | ||
| TSESTree.WhileStatement; | ||
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export default util.createRule({ | ||
name: 'strict-boolean-expressions', | ||
meta: { | ||
type: 'suggestion', | ||
docs: { | ||
description: 'Restricts the types allowed in boolean expressions', | ||
category: 'Best Practices', | ||
recommended: false, | ||
}, | ||
schema: [], | ||
messages: { | ||
strictBooleanExpression: 'Unexpected non-boolean in conditional.', | ||
}, | ||
}, | ||
defaultOptions: [], | ||
create(context) { | ||
const service = util.getParserServices(context); | ||
const checker = service.program.getTypeChecker(); | ||
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/** | ||
* Determines if the node has a boolean type. | ||
*/ | ||
function isBooleanType(node: TSESTree.Node): boolean { | ||
const tsNode = service.esTreeNodeToTSNodeMap.get<ts.ExpressionStatement>( | ||
node, | ||
); | ||
const type = util.getConstrainedTypeAtLocation(checker, tsNode); | ||
return tsutils.isTypeFlagSet(type, ts.TypeFlags.BooleanLike); | ||
} | ||
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/** | ||
* Asserts that a testable expression contains a boolean, reports otherwise. | ||
* Filters all LogicalExpressions to prevent some duplicate reports. | ||
*/ | ||
function assertTestExpressionContainsBoolean( | ||
node: ExpressionWithTest, | ||
): void { | ||
if ( | ||
node.test !== null && | ||
node.test.type !== AST_NODE_TYPES.LogicalExpression && | ||
!isBooleanType(node.test) | ||
) { | ||
reportNode(node.test); | ||
} | ||
} | ||
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/** | ||
* Asserts that a logical expression contains a boolean, reports otherwise. | ||
*/ | ||
function assertLocalExpressionContainsBoolean( | ||
node: TSESTree.LogicalExpression, | ||
): void { | ||
if (!isBooleanType(node.left) || !isBooleanType(node.right)) { | ||
reportNode(node); | ||
} | ||
} | ||
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/** | ||
* Asserts that a unary expression contains a boolean, reports otherwise. | ||
*/ | ||
function assertUnaryExpressionContainsBoolean( | ||
node: TSESTree.UnaryExpression, | ||
): void { | ||
if (!isBooleanType(node.argument)) { | ||
reportNode(node.argument); | ||
} | ||
} | ||
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/** | ||
* Reports an offending node in context. | ||
*/ | ||
function reportNode(node: TSESTree.Node): void { | ||
context.report({ node, messageId: 'strictBooleanExpression' }); | ||
} | ||
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return { | ||
ConditionalExpression: assertTestExpressionContainsBoolean, | ||
DoWhileStatement: assertTestExpressionContainsBoolean, | ||
ForStatement: assertTestExpressionContainsBoolean, | ||
IfStatement: assertTestExpressionContainsBoolean, | ||
WhileStatement: assertTestExpressionContainsBoolean, | ||
LogicalExpression: assertLocalExpressionContainsBoolean, | ||
'UnaryExpression[operator="!"]': assertUnaryExpressionContainsBoolean, | ||
}; | ||
}, | ||
}); |
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