1 files changed, 97 insertions, 0 deletions
diff --git a/tools/lint/eslint/eslint-plugin-mozilla/lib/rules/prefer-formatValues.js b/tools/lint/eslint/eslint-plugin-mozilla/lib/rules/prefer-formatValues.js
new file mode 100644
index 0000000000..29daca3c7c
--- /dev/null
+++ b/tools/lint/eslint/eslint-plugin-mozilla/lib/rules/prefer-formatValues.js
@@ -0,0 +1,97 @@
+/**
+ * @fileoverview Reject multiple calls to document.l10n.formatValue in the same
+ * code block.
+ *
+ * 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";
+
+function isIdentifier(node, id) {
+  return node && node.type === "Identifier" && node.name === id;
+}
+
+/**
+ * As we enter blocks new sets are pushed onto this stack and then popped when
+ * we exit the block.
+ */
+const BlockStack = [];
+
+module.exports = {
+  meta: {
+    docs: {
+      description: "disallow multiple document.l10n.formatValue calls",
+      url: "https://firefox-source-docs.mozilla.org/code-quality/lint/linters/eslint-plugin-mozilla/prefer-formatValues.html",
+    },
+    messages: {
+      outsideCallBlock: "call expression found outside of known block",
+      useSingleCall:
+        "prefer to use a single document.l10n.formatValues call instead " +
+        "of multiple calls to document.l10n.formatValue or document.l10n.formatValues",
+    },
+    schema: [],
+    type: "problem",
+  },
+
+  create(context) {
+    function enterBlock() {
+      BlockStack.push(new Set());
+    }
+
+    function exitBlock() {
+      let calls = BlockStack.pop();
+      if (calls.size > 1) {
+        for (let callNode of calls) {
+          context.report({
+            node: callNode,
+            messageId: "useSingleCall",
+          });
+        }
+      }
+    }
+
+    return {
+      Program: enterBlock,
+      "Program:exit": exitBlock,
+      BlockStatement: enterBlock,
+      "BlockStatement:exit": exitBlock,
+
+      CallExpression(node) {
+        if (!BlockStack.length) {
+          context.report({
+            node,
+            messageId: "outsideCallBlock",
+          });
+        }
+
+        let callee = node.callee;
+        if (callee.type !== "MemberExpression") {
+          return;
+        }
+
+        if (
+          !isIdentifier(callee.property, "formatValue") &&
+          !isIdentifier(callee.property, "formatValues")
+        ) {
+          return;
+        }
+
+        if (callee.object.type !== "MemberExpression") {
+          return;
+        }
+
+        if (
+          !isIdentifier(callee.object.object, "document") ||
+          !isIdentifier(callee.object.property, "l10n")
+        ) {
+          return;
+        }
+
+        let calls = BlockStack[BlockStack.length - 1];
+        calls.add(node);
+      },
+    };
+  },
+};