typescript-eslint · bradzacher · Jan 29, 2020 · Feb 10, 2019 · Jan 27, 2020 · Jan 27, 2020
diff --git a/packages/eslint-plugin/README.md b/packages/eslint-plugin/README.md
diff --git a/packages/eslint-plugin/ROADMAP.md b/packages/eslint-plugin/ROADMAP.md
@@ -159,7 +159,7 @@ It lists all TSLint rules along side rules from the ESLint ecosystem that are th
 | [`newline-per-chained-call`]        | 🌟  | [`newline-per-chained-call`][newline-per-chained-call]                              |
 | [`new-parens`]                      | 🌟  | [`new-parens`][new-parens]                                                          |
 | [`no-angle-bracket-type-assertion`] | ✅  | [`@typescript-eslint/consistent-type-assertions`]                                   |
-| [`no-boolean-literal-compare`]      | 🛑  | N/A                                                                                 |
+| [`no-boolean-literal-compare`]      | ✅  | [`@typescript-eslint/no-unnecessary-boolean-literal-compare`]                       |
 | [`no-consecutive-blank-lines`]      | 🌟  | [`no-multiple-empty-lines`][no-multiple-empty-lines]                                |
 | [`no-irregular-whitespace`]         | 🌟  | [`no-irregular-whitespace`][no-irregular-whitespace] with `skipStrings: false`      |
 | [`no-parameter-properties`]         | ✅  | [`@typescript-eslint/no-parameter-properties`]                                      |
@@ -600,6 +600,7 @@ Relevant plugins: [`chai-expect-keywords`](https://github.com/gavinaiken/eslint-
 [`@typescript-eslint/type-annotation-spacing`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/type-annotation-spacing.md
 [`@typescript-eslint/typedef`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/typedef.md
 [`@typescript-eslint/unified-signatures`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/unified-signatures.md
+[`@typescript-eslint/no-unnecessary-boolean-literal-compare`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-unnecessary-boolean-literal-compare.md
 [`@typescript-eslint/no-misused-new`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-misused-new.md
 [`@typescript-eslint/no-this-alias`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-this-alias.md
 [`@typescript-eslint/no-throw-literal`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-throw-literal.md

diff --git a/packages/eslint-plugin/docs/rules/no-unnecessary-boolean-literal-compare.md b/packages/eslint-plugin/docs/rules/no-unnecessary-boolean-literal-compare.md
@@ -0,0 +1,41 @@
+# Flags unnecessary equality comparisons against boolean literals (`no-unnecessary-boolean-literal-compare`)
+
+Comparing boolean values to boolean literals is unnecessary, those comparisons result in the same booleans. Using the boolean values directly, or via a unary negation (`!value`), is more concise and clearer.
+
+## Rule Details
+
+This rule ensures that you do not include unnecessary comparisons with boolean literals.
+A comparison is considered unnecessary if it checks a boolean literal against any variable with just the `boolean` type.
+A comparison is **_not_** considered unnecessary if the type is a union of booleans (`string | boolean`, `someObject | boolean`).
+
+Examples of **incorrect** code for this rule:
+
+```ts
+declare const someCondition: boolean;
+if (someCondition === true) {
+}
+```
+
+Examples of **correct** code for this rule
+
+```ts
+declare const someCondition: boolean;
+if (someCondition) {
+}
+
+declare const someObjectBoolean: boolean | Record<string, unknown>;
+if (someObjectBoolean === true) {
+}
+
+declare const someStringBoolean: boolean | string;
+if (someStringBoolean === true) {
+}
+
+declare const someUndefinedCondition: boolean | undefined;
+if (someUndefinedCondition === false) {
+}
+```
+
+## Related to
+
+- TSLint: [no-boolean-literal-compare](https://palantir.github.io/tslint/rules/no-boolean-literal-compare)
diff --git a/packages/eslint-plugin/src/configs/all.json b/packages/eslint-plugin/src/configs/all.json
@@ -53,6 +53,7 @@
     "@typescript-eslint/no-this-alias": "error",
     "@typescript-eslint/no-throw-literal": "error",
     "@typescript-eslint/no-type-alias": "error",
+    "@typescript-eslint/no-unnecessary-boolean-literal-compare": "error",
     "@typescript-eslint/no-unnecessary-condition": "error",
     "@typescript-eslint/no-unnecessary-qualifier": "error",
     "@typescript-eslint/no-unnecessary-type-arguments": "error",

diff --git a/packages/eslint-plugin/src/rules/index.ts b/packages/eslint-plugin/src/rules/index.ts
@@ -46,6 +46,7 @@ import noRequireImports from './no-require-imports';
 import noThisAlias from './no-this-alias';
 import noThrowLiteral from './no-throw-literal';
 import noTypeAlias from './no-type-alias';
+import noUnnecessaryBooleanLiteralCompare from './no-unnecessary-boolean-literal-compare';
 import noUnnecessaryCondition from './no-unnecessary-condition';
 import noUnnecessaryQualifier from './no-unnecessary-qualifier';
 import useDefaultTypeParameter from './no-unnecessary-type-arguments';
@@ -132,6 +133,7 @@ export default {
   'no-this-alias': noThisAlias,
   'no-type-alias': noTypeAlias,
   'no-throw-literal': noThrowLiteral,
+  'no-unnecessary-boolean-literal-compare': noUnnecessaryBooleanLiteralCompare,
   'no-unnecessary-condition': noUnnecessaryCondition,
   'no-unnecessary-qualifier': noUnnecessaryQualifier,
   'no-unnecessary-type-arguments': useDefaultTypeParameter,

diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-boolean-literal-compare.ts b/packages/eslint-plugin/src/rules/no-unnecessary-boolean-literal-compare.ts
@@ -0,0 +1,123 @@
+import {
+  AST_NODE_TYPES,
+  TSESTree,
+} from '@typescript-eslint/experimental-utils';
+import * as tsutils from 'tsutils';
+import * as ts from 'typescript';
+import * as util from '../util';
+
+type MessageIds = 'direct' | 'negated';
+
+interface BooleanComparison {
+  expression: TSESTree.Expression;
+  forTruthy: boolean;
+  negated: boolean;
+  range: [number, number];
+}
+
+export default util.createRule<[], MessageIds>({
+  name: 'no-unnecessary-boolean-literal-compare',
+  meta: {
+    docs: {
+      description:
+        'Flags unnecessary equality comparisons against boolean literals',
+      category: 'Stylistic Issues',
+      recommended: false,
+      requiresTypeChecking: true,
+    },
+    fixable: 'code',
+    messages: {
+      direct:
+        'This expression unnecessarily compares a boolean value to a boolean instead of using it directly',
+      negated:
+        'This expression unnecessarily compares a boolean value to a boolean instead of negating it.',
+    },
+    schema: [],
+    type: 'suggestion',
+  },
+  defaultOptions: [],
+  create(context) {
+    const parserServices = util.getParserServices(context);
+    const checker = parserServices.program.getTypeChecker();
+
+    function getBooleanComparison(
+      node: TSESTree.BinaryExpression,
+    ): BooleanComparison | undefined {
+      const comparison = deconstructComparison(node);
+      if (!comparison) {
+        return undefined;
+      }
+
+      const expressionType = checker.getTypeAtLocation(
+        parserServices.esTreeNodeToTSNodeMap.get(comparison.expression),
+      );
+
+      if (
+        !tsutils.isTypeFlagSet(
+          expressionType,
+          ts.TypeFlags.Boolean | ts.TypeFlags.BooleanLiteral,
+        )
+      ) {
+        return undefined;
+      }
+
+      return comparison;
+    }
+
+    function deconstructComparison(
+      node: TSESTree.BinaryExpression,
+    ): BooleanComparison | undefined {
+      const comparisonType = util.getEqualsKind(node.operator);
+      if (!comparisonType) {
+        return undefined;
+      }
+
+      for (const [against, expression] of [
+        [node.right, node.left],
+        [node.left, node.right],
+      ]) {
+        if (
+          against.type !== AST_NODE_TYPES.Literal ||
+          typeof against.value !== 'boolean'
+        ) {
+          continue;
+        }
+
+        const { value } = against;
+        const negated = node.operator.startsWith('!');
+
+        return {
+          forTruthy: value ? !negated : negated,
+          expression,
+          negated,
+          range:
+            expression.range[0] < against.range[0]
+              ? [expression.range[1], against.range[1]]
+              : [against.range[1], expression.range[1]],
+        };
+      }
+
+      return undefined;
+    }
+
+    return {
+      BinaryExpression(node): void {
+        const comparison = getBooleanComparison(node);
+
+        if (comparison) {
+          context.report({
+            fix: function*(fixer) {
+              yield fixer.removeRange(comparison.range);
+
+              if (!comparison.forTruthy) {
+                yield fixer.insertTextBefore(node, '!');
+              }
+            },
+            messageId: comparison.negated ? 'negated' : 'direct',
+            node,
+          });
+        }
+      },
+    };
+  },
+});
diff --git a/packages/eslint-plugin/src/util/types.ts b/packages/eslint-plugin/src/util/types.ts
@@ -254,3 +254,39 @@ export function getTokenAtPosition(
   }
   return current!;
 }
+
+export interface EqualsKind {
+  isPositive: boolean;
+  isStrict: boolean;
+}
+
+export function getEqualsKind(operator: string): EqualsKind | undefined {
+  switch (operator) {
+    case '==':
+      return {
+        isPositive: true,
+        isStrict: false,
+      };
+
+    case '===':
+      return {
+        isPositive: true,
+        isStrict: true,
+      };
+
+    case '!=':
+      return {
+        isPositive: false,
+        isStrict: false,
+      };
+
+    case '!==':
+      return {
+        isPositive: true,
+        isStrict: true,
+      };
+
+    default:
+      return undefined;
+  }
+}
diff --git a/packages/eslint-plugin/tests/rules/no-unnecessary-boolean-literal-compare.test.ts b/packages/eslint-plugin/tests/rules/no-unnecessary-boolean-literal-compare.test.ts
@@ -0,0 +1,106 @@
+import rule from '../../src/rules/no-unnecessary-boolean-literal-compare';
+import { RuleTester, getFixturesRootDir } from '../RuleTester';
+
+const rootDir = getFixturesRootDir();
+const ruleTester = new RuleTester({
+  parser: '@typescript-eslint/parser',
+  parserOptions: {
+    ecmaVersion: 2015,
+    tsconfigRootDir: rootDir,
+    project: './tsconfig.json',
+  },
+});
+
+ruleTester.run('boolean-literal-compare', rule, {
+  valid: [
+    `
+      declare const varAny: any;
+      varAny === true;
+    `,
+    `
+      declare const varAny: any;
+      varAny == false;
+    `,
+    `
+      declare const varString: string;
+      varString === false;
+    `,
+    `
+      declare const varString: string;
+      varString === true;
+    `,
+    `
+      declare const varObject: {};
+      varObject === true;
+    `,
+    `
+      declare const varObject: {};
+      varObject == false;
+    `,
+    `
+      declare const varBooleanOrString: boolean | undefined;
+      varBooleanOrString === false;
+    `,
+    `
+      declare const varBooleanOrString: boolean | undefined;
+      varBooleanOrString == true;
+    `,
+    `
+      declare const varBooleanOrUndefined: boolean | undefined;
+      varBooleanOrUndefined === true;
+    `,
+    `'false' === true;`,
+    `'true' === false;`,
+  ],
+
+  invalid: [
+    {
+      code: `true === true`,
+      errors: [
+        {
+          messageId: 'direct',
+        },
+      ],
+      output: `true`,
+    },
+    {
+      code: `false !== true`,
+      errors: [
+        {
+          messageId: 'negated',
+        },
+      ],
+      output: `!false`,
+    },
+    {
+      code: `
+        declare const varBoolean: boolean;
+        if (varBoolean !== false) { }
+      `,
+      errors: [
+        {
+          messageId: 'negated',
+        },
+      ],
+      output: `
+        declare const varBoolean: boolean;
+        if (varBoolean) { }
+      `,
+    },
+    {
+      code: `
+        declare const varTrue: true;
+        if (varTrue !== true) { }
+      `,
+      errors: [
+        {
+          messageId: 'negated',
+        },
+      ],
+      output: `
+        declare const varTrue: true;
+        if (!varTrue) { }
+      `,
+    },
+  ],
+});