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feat(eslint-plugin): add rule strict-boolean-expressions (#579)
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‎packages/eslint-plugin/README.md

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@@ -178,6 +178,7 @@ Then you should add `airbnb` (or `airbnb-base`) to your `extends` section of `.e
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| [`@typescript-eslint/require-array-sort-compare`](./docs/rules/require-array-sort-compare.md) | Enforce giving `compare` argument to `Array#sort` | | | :thought_balloon: |
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| [`@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: |
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| [`@typescript-eslint/semi`](./docs/rules/semi.md) | Require or disallow semicolons instead of ASI | | :wrench: | |
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| [`@typescript-eslint/strict-boolean-expressions`](./docs/rules/strict-boolean-expressions.md) | Restricts the types allowed in boolean expressions | | | :thought_balloon: |
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| [`@typescript-eslint/type-annotation-spacing`](./docs/rules/type-annotation-spacing.md) | Require consistent spacing around type annotations | :heavy_check_mark: | :wrench: | |
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| [`@typescript-eslint/unbound-method`](./docs/rules/unbound-method.md) | Enforces unbound methods are called with their expected scope | | | :thought_balloon: |
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| [`@typescript-eslint/unified-signatures`](./docs/rules/unified-signatures.md) | Warns for any two overloads that could be unified into one by using a union or an optional/rest parameter | | | |

‎packages/eslint-plugin/ROADMAP.md

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| [`prefer-object-spread`] | 🌟 | [`prefer-object-spread`][prefer-object-spread] |
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| [`radix`] | 🌟 | [`radix`][radix] |
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| [`restrict-plus-operands`] | ✅ | [`@typescript-eslint/restrict-plus-operands`] |
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| [`strict-boolean-expressions`] | 🛑 | N/A |
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| [`strict-boolean-expressions`] | ✅ | [`@typescript-eslint/strict-boolean-expressions`] |
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| [`strict-type-predicates`] | 🛑 | N/A |
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| [`switch-default`] | 🌟 | [`default-case`][default-case] |
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| [`triple-equals`] | 🌟 | [`eqeqeq`][eqeqeq] |

‎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
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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
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number was defined. Doing `if (number)` would evaluate to `false` if `number`
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was defined and `0`. This rule forces these expressions to be explicit and to
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strictly use booleans.
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The following nodes are checked:
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- Arguments to the `!`, `&&`, and `||` operators
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- The condition in a conditional expression `(cond ? x : y)`
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- 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
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const number = 0;
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if (number) {
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return;
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}
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let foo = bar || 'foobar';
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let undefinedItem;
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let foo = undefinedItem ? 'foo' : 'bar';
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let str = 'foo';
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while (str) {
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break;
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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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const number = 0;
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if (typeof number !== 'undefined') {
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return;
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}
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let foo = typeof bar !== 'undefined' ? bar : 'foobar';
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let undefinedItem;
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let foo = typeof undefinedItem !== 'undefined' ? 'foo' : 'bar';
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let str = 'foo';
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while (typeof str !== 'undefined') {
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break;
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}
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```
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## Related To
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- TSLint: [strict-boolean-expressions](https://palantir.github.io/tslint/rules/strict-boolean-expressions)

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

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@@ -53,6 +53,7 @@ import promiseFunctionAsync from './promise-function-async';
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import requireArraySortCompare from './require-array-sort-compare';
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import restrictPlusOperands from './restrict-plus-operands';
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import semi from './semi';
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import strictBooleanExpressions from './strict-boolean-expressions';
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import typeAnnotationSpacing from './type-annotation-spacing';
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import unboundMethod from './unbound-method';
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import unifiedSignatures from './unified-signatures';
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'require-array-sort-compare': requireArraySortCompare,
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'restrict-plus-operands': restrictPlusOperands,
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semi: semi,
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'strict-boolean-expressions': strictBooleanExpressions,
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'type-annotation-spacing': typeAnnotationSpacing,
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'unbound-method': unboundMethod,
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'unified-signatures': unifiedSignatures,

‎packages/eslint-plugin/src/rules/strict-boolean-expressions.ts

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import {
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TSESTree,
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AST_NODE_TYPES,
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} from '@typescript-eslint/experimental-utils';
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import ts from 'typescript';
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import * as tsutils from 'tsutils';
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import * as util from '../util';
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type ExpressionWithTest =
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| TSESTree.ConditionalExpression
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| TSESTree.DoWhileStatement
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| TSESTree.ForStatement
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| TSESTree.IfStatement
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| TSESTree.WhileStatement;
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export default util.createRule({
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name: 'strict-boolean-expressions',
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meta: {
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type: 'suggestion',
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docs: {
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description: 'Restricts the types allowed in boolean expressions',
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category: 'Best Practices',
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recommended: false,
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},
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schema: [],
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messages: {
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strictBooleanExpression: 'Unexpected non-boolean in conditional.',
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},
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},
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defaultOptions: [],
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create(context) {
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const service = util.getParserServices(context);
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const checker = service.program.getTypeChecker();
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/**
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* Determines if the node has a boolean type.
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*/
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function isBooleanType(node: TSESTree.Node): boolean {
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const tsNode = service.esTreeNodeToTSNodeMap.get<ts.ExpressionStatement>(
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node,
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);
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const type = util.getConstrainedTypeAtLocation(checker, tsNode);
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return tsutils.isTypeFlagSet(type, ts.TypeFlags.BooleanLike);
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}
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/**
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* Asserts that a testable expression contains a boolean, reports otherwise.
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* Filters all LogicalExpressions to prevent some duplicate reports.
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*/
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function assertTestExpressionContainsBoolean(
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node: ExpressionWithTest,
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): void {
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if (
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node.test !== null &&
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node.test.type !== AST_NODE_TYPES.LogicalExpression &&
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!isBooleanType(node.test)
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) {
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reportNode(node.test);
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}
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}
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/**
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* Asserts that a logical expression contains a boolean, reports otherwise.
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*/
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function assertLocalExpressionContainsBoolean(
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node: TSESTree.LogicalExpression,
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): void {
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if (!isBooleanType(node.left) || !isBooleanType(node.right)) {
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reportNode(node);
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}
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}
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/**
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* Asserts that a unary expression contains a boolean, reports otherwise.
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*/
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function assertUnaryExpressionContainsBoolean(
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node: TSESTree.UnaryExpression,
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): void {
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if (!isBooleanType(node.argument)) {
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reportNode(node.argument);
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}
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}
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/**
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* Reports an offending node in context.
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*/
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function reportNode(node: TSESTree.Node): void {
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context.report({ node, messageId: 'strictBooleanExpression' });
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}
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return {
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ConditionalExpression: assertTestExpressionContainsBoolean,
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DoWhileStatement: assertTestExpressionContainsBoolean,
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ForStatement: assertTestExpressionContainsBoolean,
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IfStatement: assertTestExpressionContainsBoolean,
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WhileStatement: assertTestExpressionContainsBoolean,
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LogicalExpression: assertLocalExpressionContainsBoolean,
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'UnaryExpression[operator="!"]': assertUnaryExpressionContainsBoolean,
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};
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},
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});

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