feat(eslint-plugin): [no-unnece-cond] Add allowConstantLoopConditions (typescript-eslint#1029)

a-tarasyuk · bradzacher · bradzacher · commit ceb6f1ce225a · 2019-11-21T09:17:50.000-08:00
Co-authored-by: Brad Zacher &lt;brad.zacher@gmail.com&gt;
diff --git a/packages/eslint-plugin/docs/rules/no-unnecessary-condition.md b/packages/eslint-plugin/docs/rules/no-unnecessary-condition.md
@@ -53,6 +53,16 @@ function head<T>(items: T[]) {
 }
 ```
 
+- `allowConstantLoopConditions` (default `false`) - allows constant expressions in loops.
+
+Example of correct code for when `allowConstantLoopConditions` is `true`:
+
+```ts
+while (true) {}
+for (; true; ) {}
+do {} while (true);
+```
+
 ## When Not To Use It
 
 The main downside to using this rule is the need for type information.
diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-condition.ts b/packages/eslint-plugin/src/rules/no-unnecessary-condition.ts
@@ -41,15 +41,9 @@ const isLiteral = (type: ts.Type): boolean =>
   isLiteralType(type);
 // #endregion
 
-type ExpressionWithTest =
-  | TSESTree.ConditionalExpression
-  | TSESTree.DoWhileStatement
-  | TSESTree.ForStatement
-  | TSESTree.IfStatement
-  | TSESTree.WhileStatement;
-
 export type Options = [
   {
+    allowConstantLoopConditions?: boolean;
     ignoreRhs?: boolean;
   },
 ];
@@ -74,6 +68,9 @@ export default createRule<Options, MessageId>({
       {
         type: 'object',
         properties: {
+          allowConstantLoopConditions: {
+            type: 'boolean',
+          },
           ignoreRhs: {
             type: 'boolean',
           },
@@ -91,10 +88,11 @@ export default createRule<Options, MessageId>({
   },
   defaultOptions: [
     {
+      allowConstantLoopConditions: false,
       ignoreRhs: false,
     },
   ],
-  create(context, [{ ignoreRhs }]) {
+  create(context, [{ allowConstantLoopConditions, ignoreRhs }]) {
     const service = getParserServices(context);
     const checker = service.program.getTypeChecker();
 
@@ -165,14 +163,13 @@ export default createRule<Options, MessageId>({
      * Filters all LogicalExpressions to prevent some duplicate reports.
      */
     function checkIfTestExpressionIsNecessaryConditional(
-      node: ExpressionWithTest,
+      node: TSESTree.ConditionalExpression | TSESTree.IfStatement,
     ): void {
-      if (
-        node.test !== null &&
-        node.test.type !== AST_NODE_TYPES.LogicalExpression
-      ) {
-        checkNode(node.test);
+      if (node.test.type === AST_NODE_TYPES.LogicalExpression) {
+        return;
       }
+
+      checkNode(node.test);
     }
 
     /**
@@ -187,14 +184,46 @@ export default createRule<Options, MessageId>({
       }
     }
 
+    /**
+     * Checks that a testable expression of a loop is necessarily conditional, reports otherwise.
+     */
+    function checkIfLoopIsNecessaryConditional(
+      node:
+        | TSESTree.DoWhileStatement
+        | TSESTree.ForStatement
+        | TSESTree.WhileStatement,
+    ): void {
+      if (
+        node.test === null ||
+        node.test.type === AST_NODE_TYPES.LogicalExpression
+      ) {
+        return;
+      }
+
+      /**
+       * Allow:
+       *   while (true) {}
+       *   for (;true;) {}
+       *   do {} while (true)
+       */
+      if (
+        allowConstantLoopConditions &&
+        isBooleanLiteralType(getNodeType(node.test), true)
+      ) {
+        return;
+      }
+
+      checkNode(node.test);
+    }
+
     return {
       BinaryExpression: checkIfBinaryExpressionIsNecessaryConditional,
       ConditionalExpression: checkIfTestExpressionIsNecessaryConditional,
-      DoWhileStatement: checkIfTestExpressionIsNecessaryConditional,
-      ForStatement: checkIfTestExpressionIsNecessaryConditional,
+      DoWhileStatement: checkIfLoopIsNecessaryConditional,
+      ForStatement: checkIfLoopIsNecessaryConditional,
       IfStatement: checkIfTestExpressionIsNecessaryConditional,
-      WhileStatement: checkIfTestExpressionIsNecessaryConditional,
       LogicalExpression: checkLogicalExpressionForUnnecessaryConditionals,
+      WhileStatement: checkIfLoopIsNecessaryConditional,
     };
   },
 });
diff --git a/packages/eslint-plugin/tests/rules/no-unnecessary-condition.test.ts b/packages/eslint-plugin/tests/rules/no-unnecessary-condition.test.ts
@@ -97,6 +97,14 @@ declare const b2: true;
 if(b1 && b2) {}`,
       options: [{ ignoreRhs: true }],
     },
+    {
+      code: `
+while(true) {}
+for (;true;) {}
+do {} while(true)
+      `,
+      options: [{ allowConstantLoopConditions: true }],
+    },
   ],
   invalid: [
     // Ensure that it's checking in all the right places
@@ -201,5 +209,18 @@ const t1 = (b1 && b2) ? 'yes' : 'no'`,
         ruleError(9, 13, 'alwaysTruthy'),
       ],
     },
+    {
+      code: `
+while(true) {}
+for (;true;) {}
+do {} while(true)
+      `,
+      options: [{ allowConstantLoopConditions: false }],
+      errors: [
+        ruleError(2, 7, 'alwaysTruthy'),
+        ruleError(3, 7, 'alwaysTruthy'),
+        ruleError(4, 13, 'alwaysTruthy'),
+      ],
+    },
   ],
 });
