feat: correct update direction in for-direction (#17483)

* feat: correct update direction in `for-direction`

* Improve documentation

* Update rule overview

Co-authored-by: Nicholas C. Zakas <nicholas@humanwhocodes.com>

* Improve formatting

---------

Co-authored-by: Nicholas C. Zakas <nicholas@humanwhocodes.com>

docs/src/rules/for-direction.md

@@ -3,11 +3,11 @@ title: for-direction
 rule_type: problem
 ---
 
-
+A `for` loop with a stop condition that can never be reached, such as one with a counter that moves in the wrong direction, will run infinitely. While there are occasions when an infinite loop is intended, the convention is to construct such loops as `while` loops. More typically, an infinite `for` loop is a bug.
 
 ## Rule Details
 
-A `for` loop with a stop condition that can never be reached, such as one with a counter that moves in the wrong direction, will run infinitely. While there are occasions when an infinite loop is intended, the convention is to construct such loops as `while` loops. More typically, an infinite for loop is a bug.
+This rule forbids `for` loops where the counter variable changes in such a way that the stop condition will never be met. For example, if the counter variable is increasing (i.e. `i++`) and the stop condition tests that the counter is greater than zero (`i >= 0`) then the loop will never exit.
 
 Examples of **incorrect** code for this rule:
 
@@ -23,6 +23,10 @@ for (var i = 10; i >= 0; i++) {
 
 for (var i = 0; i > 10; i++) {
 }
+
+const n = -2;
+for (let i = 0; i < 10; i += n) {
+}
 ```
 
 :::
@@ -35,6 +39,12 @@ Examples of **correct** code for this rule:
 /*eslint for-direction: "error"*/
 for (var i = 0; i < 10; i++) {
 }
+
+for (let i = 10; i >= 0; i += this.step) { // direction unknown
+}
+
+for (let i = MIN; i <= MAX; i -= 0) { // not increasing or decreasing
+}
 ```
 
 :::

lib/rules/for-direction.js

@@ -5,6 +5,12 @@
 
 "use strict";
 
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+const { getStaticValue } = require("@eslint-community/eslint-utils");
+
 //------------------------------------------------------------------------------
 // Rule Definition
 //------------------------------------------------------------------------------
@@ -29,6 +35,7 @@ module.exports = {
     },
 
     create(context) {
+        const { sourceCode } = context;
 
         /**
          * report an error.
@@ -46,17 +53,17 @@ module.exports = {
          * check the right side of the assignment
          * @param {ASTNode} update UpdateExpression to check
          * @param {int} dir expected direction that could either be turned around or invalidated
-         * @returns {int} return dir, the negated dir or zero if it's not clear for identifiers
+         * @returns {int} return dir, the negated dir, or zero if the counter does not change or the direction is not clear
          */
         function getRightDirection(update, dir) {
-            if (update.right.type === "UnaryExpression") {
-                if (update.right.operator === "-") {
-                    return -dir;
-                }
-            } else if (update.right.type === "Identifier") {
-                return 0;
+            const staticValue = getStaticValue(update.right, sourceCode.getScope(update));
+
+            if (staticValue && ["bigint", "boolean", "number"].includes(typeof staticValue.value)) {
+                const sign = Math.sign(Number(staticValue.value)) || 0; // convert NaN to 0
+
+                return dir * sign;
             }
-            return dir;
+            return 0;
         }
 
         /**

tests/lib/rules/for-direction.js

@@ -16,7 +16,7 @@ const { RuleTester } = require("../../../lib/rule-tester");
 // Tests
 //------------------------------------------------------------------------------
 
-const ruleTester = new RuleTester();
+const ruleTester = new RuleTester({ parserOptions: { ecmaVersion: 2020 } });
 const incorrectDirection = { messageId: "incorrectDirection" };
 
 ruleTester.run("for-direction", rule, {
@@ -37,6 +37,12 @@ ruleTester.run("for-direction", rule, {
         "for(var i = 10; i >= 0; i-=1){}",
         "for(var i = 10; i > 0; i+=-1){}",
         "for(var i = 10; i >= 0; i+=-1){}",
+        "for(var i = 0n; i > l; i-=1n){}",
+        "for(var i = 0n; i < l; i-=-1n){}",
+        "for(var i = MIN; i <= MAX; i+=true){}",
+        "for(var i = 0; i < 10; i+=+5e-7){}",
+        "for(var i = 0; i < MAX; i -= ~2);",
+        "for(var i = 0, n = -1; i < MAX; i += -n);",
 
         // test if no update.
         "for(var i = 10; i > 0;){}",
@@ -54,6 +60,13 @@ ruleTester.run("for-direction", rule, {
         "for(var i = 0; i < MAX; i += STEP_SIZE);",
         "for(var i = 0; i < MAX; i -= STEP_SIZE);",
         "for(var i = 10; i > 0; i += STEP_SIZE);",
+        "for(var i = 10; i >= 0; i += 0);",
+        "for(var i = 10n; i >= 0n; i += 0n);",
+        "for(var i = 10; i >= 0; i += this.step);",
+        "for(var i = 10; i >= 0; i += 'foo');",
+        "for(var i = 10; i > 0; i += !foo);",
+        "for(var i = MIN; i <= MAX; i -= false);",
+        "for(var i = MIN; i <= MAX; i -= 0/0);",
 
         // other cond-expressions.
         "for(var i = 0; i !== 10; i+=1){}",
@@ -77,6 +90,12 @@ ruleTester.run("for-direction", rule, {
         { code: "for(var i = 0; i < 10; i+=-1){}", errors: [incorrectDirection] },
         { code: "for(var i = 0; i <= 10; i+=-1){}", errors: [incorrectDirection] },
         { code: "for(var i = 10; i > 10; i-=-1){}", errors: [incorrectDirection] },
-        { code: "for(var i = 10; i >= 0; i-=-1){}", errors: [incorrectDirection] }
+        { code: "for(var i = 10; i >= 0; i-=-1){}", errors: [incorrectDirection] },
+        { code: "for(var i = 0n; i > l; i+=1n){}", errors: [incorrectDirection] },
+        { code: "for(var i = 0n; i < l; i+=-1n){}", errors: [incorrectDirection] },
+        { code: "for(var i = MIN; i <= MAX; i-=true){}", errors: [incorrectDirection] },
+        { code: "for(var i = 0; i < 10; i-=+5e-7){}", errors: [incorrectDirection] },
+        { code: "for(var i = 0; i < MAX; i += (2 - 3));", errors: [incorrectDirection] },
+        { code: "var n = -2; for(var i = 0; i < 10; i += n);", errors: [incorrectDirection] }
     ]
 });