Fix: no-plusplus allow comma operands in for afterthought (fixes #13005)

docs/rules/no-plusplus.md

@@ -71,10 +71,30 @@ Examples of **correct** code for this rule with the `{ "allowForLoopAfterthought
 /*eslint no-plusplus: ["error", { "allowForLoopAfterthoughts": true }]*/
 
 for (i = 0; i < l; i++) {
-    return;
+    doSomething(i);
 }
 
-for (i = 0; i < l; i--) {
-    return;
+for (i = l; i >= 0; i--) {
+    doSomething(i);
+}
+
+for (i = 0, j = l; i < l; i++, j--) {
+    doSomething(i, j);
+}
+```
+
+Examples of **incorrect** code for this rule with the `{ "allowForLoopAfterthoughts": true }` option:
+
+```js
+/*eslint no-plusplus: ["error", { "allowForLoopAfterthoughts": true }]*/
+
+for (i = 0; i < l; j = i++) {
+    doSomething(i, j);
 }
+
+for (i = l; i--;) {
+    doSomething(i);
+}
+
+for (i = 0; i < l;) i++;
 ```

lib/rules/no-plusplus.js

@@ -6,6 +6,34 @@
 
 "use strict";
 
+//------------------------------------------------------------------------------
+// Helpers
+//------------------------------------------------------------------------------
+
+/**
+ * Determines whether the given node is the update node of a `ForStatement`.
+ * @param {ASTNode} node The node to check.
+ * @returns {boolean} `true` if the node is `ForStatement` update.
+ */
+function isForStatementUpdate(node) {
+    const parent = node.parent;
+
+    return parent.type === "ForStatement" && parent.update === node;
+}
+
+/**
+ * Determines whether the given node is considered to be a for loop "afterthought" by the logic of this rule.
+ * In particular, if it is the update node of a `ForStatement`, or an operand of a sequence expression that is the update node.
+ * @param {ASTNode} node The node to check.
+ * @returns {boolean} `true` if the node is a for loop afterthought.
+ */
+function isForLoopAfterthought(node) {
+    const parent = node.parent;
+
+    return isForStatementUpdate(node) ||
+        parent.type === "SequenceExpression" && isForStatementUpdate(parent);
+}
+
 //------------------------------------------------------------------------------
 // Rule Definition
 //------------------------------------------------------------------------------
@@ -42,18 +70,19 @@ module.exports = {
     create(context) {
 
         const config = context.options[0];
-        let allowInForAfterthought = false;
+        let allowForLoopAfterthoughts = false;
 
         if (typeof config === "object") {
-            allowInForAfterthought = config.allowForLoopAfterthoughts === true;
+            allowForLoopAfterthoughts = config.allowForLoopAfterthoughts === true;
         }
 
         return {
 
             UpdateExpression(node) {
-                if (allowInForAfterthought && node.parent.type === "ForStatement") {
+                if (allowForLoopAfterthoughts && isForLoopAfterthought(node)) {
                     return;
                 }
+
                 context.report({
                     node,
                     messageId: "unexpectedUnaryOp",

tests/lib/rules/no-plusplus.js

@@ -24,7 +24,11 @@ ruleTester.run("no-plusplus", rule, {
 
         // With "allowForLoopAfterthoughts" allowed
         { code: "var foo = 0; foo=+1;", options: [{ allowForLoopAfterthoughts: true }] },
-        { code: "for (i = 0; i < l; i++) { console.log(i); }", options: [{ allowForLoopAfterthoughts: true }] }
+        { code: "for (i = 0; i < l; i++) { console.log(i); }", options: [{ allowForLoopAfterthoughts: true }] },
+        { code: "for (var i = 0, j = i + 1; j < example.length; i++, j++) {}", options: [{ allowForLoopAfterthoughts: true }] },
+        { code: "for (;; i--, foo());", options: [{ allowForLoopAfterthoughts: true }] },
+        { code: "for (;; foo(), --i);", options: [{ allowForLoopAfterthoughts: true }] },
+        { code: "for (;; foo(), ++i, bar);", options: [{ allowForLoopAfterthoughts: true }] }
     ],
 
     invalid: [
@@ -81,6 +85,61 @@ ruleTester.run("no-plusplus", rule, {
                 },
                 type: "UpdateExpression"
             }]
+        },
+        {
+            code: "for (i++;;);",
+            options: [{ allowForLoopAfterthoughts: true }],
+            errors: [{
+                messageId: "unexpectedUnaryOp",
+                data: {
+                    operator: "++"
+                },
+                type: "UpdateExpression"
+            }]
+        },
+        {
+            code: "for (;--i;);",
+            options: [{ allowForLoopAfterthoughts: true }],
+            errors: [{
+                messageId: "unexpectedUnaryOp",
+                data: {
+                    operator: "--"
+                },
+                type: "UpdateExpression"
+            }]
+        },
+        {
+            code: "for (;;) ++i;",
+            options: [{ allowForLoopAfterthoughts: true }],
+            errors: [{
+                messageId: "unexpectedUnaryOp",
+                data: {
+                    operator: "++"
+                },
+                type: "UpdateExpression"
+            }]
+        },
+        {
+            code: "for (;; i = j++);",
+            options: [{ allowForLoopAfterthoughts: true }],
+            errors: [{
+                messageId: "unexpectedUnaryOp",
+                data: {
+                    operator: "++"
+                },
+                type: "UpdateExpression"
+            }]
+        },
+        {
+            code: "for (;; i++, f(--j));",
+            options: [{ allowForLoopAfterthoughts: true }],
+            errors: [{
+                messageId: "unexpectedUnaryOp",
+                data: {
+                    operator: "--"
+                },
+                type: "UpdateExpression"
+            }]
         }
     ]
 });