[feat] add no-negated-condition-rule

src/rules/terNoNegatedConditionRule.ts

@@ -0,0 +1,70 @@
+import * as ts from 'typescript';
+import * as Lint from 'tslint';
+
+const RULE_NAME = 'ter-no-negated-condition';
+export class Rule extends Lint.Rules.AbstractRule {
+  public static FAILURE_STRING = 'Unexpected negated condition.';
+  public static metadata: Lint.IRuleMetadata = {
+    ruleName: RULE_NAME,
+    hasFix: false,
+    description: 'Disallows negated conditions for if statements with else branch and ternary expressions',
+    rationale: Lint.Utils.dedent`
+       Negated conditions are more difficult to understand.
+       Code can be made more readable by inverting the condition instead.`,
+    optionsDescription: '',
+    options: {},
+    optionExamples: [
+      Lint.Utils.dedent`
+        "${RULE_NAME}": true
+        `
+    ],
+    typescriptOnly: false,
+    type: 'style'
+  };
+
+  public apply(sourceFile: ts.SourceFile): Lint.RuleFailure[] {
+    return this.applyWithWalker(new Walk(sourceFile, this.getOptions()));
+  }
+}
+
+class Walk extends Lint.RuleWalker {
+  protected visitIfStatement(node: ts.IfStatement) {
+    if (this.hasElseWithoutCondition(node) && (
+        this.isSingleOperatorNegatedCondition(node.expression) ? this.isConditionNegated(node.expression) : this.isConditionNotEqual(node.expression as ts.BinaryExpression)
+      )) {
+      this.addFailureAtNode(
+        node,
+        Rule.FAILURE_STRING
+      );
+    }
+    super.visitIfStatement(node);
+  }
+
+  protected visitConditionalExpression(node: ts.ConditionalExpression) {
+    if ((this.isSingleOperatorNegatedCondition(node.condition) && this.isConditionNegated(node.condition)) || (this.isConditionNotEqual(node.condition as ts.BinaryExpression))) {
+      this.addFailureAtNode(
+        node,
+        Rule.FAILURE_STRING
+      );
+    }
+    super.visitConditionalExpression(node);
+  }
+
+  private hasElseWithoutCondition(node: ts.IfStatement) {
+    return node.elseStatement && node.elseStatement.getFirstToken() && node.elseStatement.getFirstToken()!.kind !== ts.SyntaxKind.IfKeyword;
+  }
+
+  private isSingleOperatorNegatedCondition(condition: ts.ConditionalExpression['condition']) {
+    // Child count should be 2 so we can ensure that whole condition is negated(e.g. !(true && true) instead of !true && false)
+    return condition.getChildCount() === 2;
+  }
+
+  private isConditionNegated(condition: ts.ConditionalExpression['condition']) {
+    return condition.getFirstToken() && condition.getFirstToken()!.kind === ts.SyntaxKind.ExclamationToken;
+  }
+
+  private isConditionNotEqual(condition: ts.BinaryExpression) {
+    return condition.operatorToken && (condition.operatorToken.kind === ts.SyntaxKind.ExclamationEqualsToken ||
+                                       condition.operatorToken.kind === ts.SyntaxKind.ExclamationEqualsEqualsToken);
+  }
+}

src/test/rules/terNoNegatedConditionRuleTests.ts

@@ -0,0 +1,56 @@
+import { RuleTester, Position } from './ruleTester';
+// ESLint Tests: https://github.com/eslint/eslint/blob/master/lib/rules/no-negated-condition.js
+
+const ruleTester = new RuleTester('ter-no-negated-condition');
+
+const noNegatedConditionError = {
+  failure: 'Unexpected negated condition.',
+  startPosition: new Position(0),
+  endPosition: new Position(0)
+};
+
+ruleTester.addTestGroup('valid', 'should pass when condition is not negated or cannot be converted', [
+  'if (a) {}',
+  'if (a) {} else {}',
+  'if (!a) {}',
+  'if (!a) {} else if (b) {}',
+  'if (!a) {} else if (b) {} else {}',
+  'if (a == b) {}',
+  'if (a == b) {} else {}',
+  'if (a != b) {}',
+  'if (a != b) {} else if (b) {}',
+  'if (a != b) {} else if (b) {} else {}',
+  'if (a !== b) {}',
+  'if (a === b) {} else {}',
+  'a ? b : c',
+  '!a && b ? c : d'
+]);
+
+ruleTester.addTestGroup('invalid', 'should fail condition is negated and second branch is available', [
+  {
+    code: 'if (!a) {;} else {;}',
+    errors: [noNegatedConditionError]
+  },
+  {
+    code: 'if (a != b) {;} else {;}',
+    errors: [noNegatedConditionError]
+  },
+  {
+    code: 'if (a !== b) {;} else {;}',
+    errors: [noNegatedConditionError]
+  },
+  {
+    code: '!a ? b : c',
+    errors: [noNegatedConditionError]
+  },
+  {
+    code: 'a != b ? c : d',
+    errors: [noNegatedConditionError]
+  },
+  {
+    code: 'a !== b ? c : d',
+    errors: [noNegatedConditionError]
+  }
+]);
+
+ruleTester.runTests();