1 files changed, 41 insertions, 0 deletions
diff --git a/tools/lint/eslint/eslint-plugin-mozilla/lib/rules/use-returnValue.js b/tools/lint/eslint/eslint-plugin-mozilla/lib/rules/use-returnValue.js
new file mode 100644
index 0000000000..29722a4f49
--- /dev/null
+++ b/tools/lint/eslint/eslint-plugin-mozilla/lib/rules/use-returnValue.js
@@ -0,0 +1,41 @@
+/**
+ * @fileoverview Warn when idempotent methods are called and their return value is unused.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+"use strict";
+
+module.exports = {
+  meta: {
+    docs: {
+      url: "https://firefox-source-docs.mozilla.org/code-quality/lint/linters/eslint-plugin-mozilla/use-returnValue.html",
+    },
+    schema: [],
+    type: "problem",
+  },
+
+  create(context) {
+    return {
+      ExpressionStatement(node) {
+        if (
+          node.expression?.type != "CallExpression" ||
+          node.expression.callee?.type != "MemberExpression" ||
+          node.expression.callee.property?.type != "Identifier" ||
+          !["concat", "join", "slice"].includes(
+            node.expression.callee.property?.name
+          )
+        ) {
+          return;
+        }
+
+        context.report({
+          node,
+          message: `{Array/String}.${node.expression.callee.property.name} doesn't modify the instance in-place`,
+        });
+      },
+    };
+  },
+};