Apply the small-value-space guard to null-valued constants in no-duplicate-constant-values

eslint-factory/README.md

@@ -63,7 +63,7 @@ This project hosts custom ESLint linters for `/actions/setup/js`.
 
 Inventory module-level `const` declarations with static primitive initializers and report each declaration after the first one that uses the same value in a file. The diagnostic names both constants and shows the duplicated value.
 
-The rule compares string, number, boolean, `null`, bigint, regular-expression, static template-literal, and signed numeric initializers. To avoid collisions in their small value spaces, it reports duplicate numeric and boolean values only when at least three module-level constants share them. Dynamic expressions, object and array literals, destructuring declarations, function-local declarations, and `let` or `var` declarations are ignored.
+The rule compares string, number, boolean, `null`, bigint, regular-expression, static template-literal, and signed numeric initializers. To avoid collisions in their small value spaces, it reports duplicate numeric, boolean, and `null` values only when at least three module-level constants share them. Dynamic expressions, object and array literals, destructuring declarations, function-local declarations, and `let` or `var` declarations are ignored.
 
 ### `no-github-request-interpolated-route`
 

eslint-factory/src/rules/no-duplicate-constant-values.test.ts

@@ -29,6 +29,7 @@ describe("no-duplicate-constant-values", () => {
         `const DEFAULT_HTTP_TIMEOUT_MS = 15000; const TOOL_CALL_TIMEOUT_BUFFER_MS = 15000; const NOTIFY_TIMEOUT_MS = 10000; const KEEPALIVE_PING_INTERVAL_MS = 10000;`,
         `const FIRST_ENABLED = true; const SECOND_ENABLED = true;`,
         `const FIRST_ENABLED = false; const SECOND_ENABLED = false;`,
+        `const FIRST_VALUE = null; const SECOND_VALUE = null;`,
       ],
       invalid: [],
     });
@@ -87,6 +88,21 @@ describe("no-duplicate-constant-values", () => {
     });
   });
 
+  it("requires at least three matching null constants before reporting duplicates", () => {
+    ruleTester.run("no-duplicate-constant-values", noDuplicateConstantValuesRule, {
+      valid: [`const FIRST_VALUE = null; const SECOND_VALUE = null;`],
+      invalid: [
+        {
+          code: `const FIRST_VALUE = null; const SECOND_VALUE = null; const THIRD_VALUE = null;`,
+          errors: [
+            { messageId: "duplicateConstantValue", data: { name: "SECOND_VALUE", originalName: "FIRST_VALUE", value: "null" } },
+            { messageId: "duplicateConstantValue", data: { name: "THIRD_VALUE", originalName: "FIRST_VALUE", value: "null" } },
+          ],
+        },
+      ],
+    });
+  });
+
   it("reports every duplicate after the first declaration", () => {
     ruleTester.run("no-duplicate-constant-values", noDuplicateConstantValuesRule, {
       valid: [],

eslint-factory/src/rules/no-duplicate-constant-values.ts

@@ -12,6 +12,11 @@ const MIN_NUMERIC_DUPLICATE_GROUP_SIZE = 3;
 // so unrelated constants coincidentally sharing `true`/`false` are at least as likely as the
 // numeric case above. Apply the same minimum-group-size guard to avoid false positives.
 const MIN_BOOLEAN_DUPLICATE_GROUP_SIZE = 3;
+// `null` has exactly 1 possible value, an even smaller space than booleans, so unrelated
+// placeholder or not-yet-initialized constants coincidentally sharing `null` are at least as
+// likely as the boolean case above. Apply the same minimum-group-size guard.
+const MIN_NULL_DUPLICATE_GROUP_SIZE = 3;
+const NULL_VALUE_KEY = "object:null";
 
 function getStaticValueKey(node: TSESTree.Expression): string | null {
   if (node.type === AST_NODE_TYPES.Literal) {
@@ -32,6 +37,13 @@ function getStaticValueKey(node: TSESTree.Expression): string | null {
   return null;
 }
 
+function getMinDuplicateGroupSize(valueKey: string): number {
+  if (valueKey.startsWith("number:")) return MIN_NUMERIC_DUPLICATE_GROUP_SIZE;
+  if (valueKey.startsWith("boolean:")) return MIN_BOOLEAN_DUPLICATE_GROUP_SIZE;
+  if (valueKey === NULL_VALUE_KEY) return MIN_NULL_DUPLICATE_GROUP_SIZE;
+  return 2;
+}
+
 export const noDuplicateConstantValuesRule = createRule({
   name: "no-duplicate-constant-values",
   meta: {
@@ -83,7 +95,7 @@ export const noDuplicateConstantValuesRule = createRule({
       },
       "Program:exit"() {
         for (const [valueKey, declarations] of constantsByValue) {
-          const minGroupSize = valueKey.startsWith("number:") ? MIN_NUMERIC_DUPLICATE_GROUP_SIZE : valueKey.startsWith("boolean:") ? MIN_BOOLEAN_DUPLICATE_GROUP_SIZE : 2;
+          const minGroupSize = getMinDuplicateGroupSize(valueKey);
           const shouldReportDuplicates = declarations.length >= minGroupSize;
           if (!shouldReportDuplicates) continue;