1 files changed, 74 insertions, 0 deletions
diff --git a/tools/eslint/lib/rules/no-unused-expressions.js b/tools/eslint/lib/rules/no-unused-expressions.js
new file mode 100644
index 0000000000..7036f90e7c
--- /dev/null
+++ b/tools/eslint/lib/rules/no-unused-expressions.js
@@ -0,0 +1,74 @@
+/**
+ * @fileoverview Flag expressions in statement position that do not side effect
+ * @author Michael Ficarra
+ * @copyright 2013 Michael Ficarra. All rights reserved.
+ */
+"use strict";
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = function(context) {
+
+    /**
+     * @param {ASTNode} node - any node
+     * @returns {Boolean} whether the given node structurally represents a directive
+     */
+    function looksLikeDirective(node) {
+        return node.type === "ExpressionStatement" &&
+            node.expression.type === "Literal" && typeof node.expression.value === "string";
+    }
+
+    /**
+     * @param {Function} predicate - ([a] -> Boolean) the function used to make the determination
+     * @param {a[]} list - the input list
+     * @returns {a[]} the leading sequence of members in the given list that pass the given predicate
+     */
+    function takeWhile(predicate, list) {
+        for (var i = 0, l = list.length; i < l; ++i) {
+            if (!predicate(list[i])) {
+                break;
+            }
+        }
+        return [].slice.call(list, 0, i);
+    }
+
+    /**
+     * @param {ASTNode} node - a Program or BlockStatement node
+     * @returns {ASTNode[]} the leading sequence of directive nodes in the given node's body
+     */
+    function directives(node) {
+        return takeWhile(looksLikeDirective, node.body);
+    }
+
+    /**
+     * @param {ASTNode} node - any node
+     * @param {ASTNode[]} ancestors - the given node's ancestors
+     * @returns {Boolean} whether the given node is considered a directive in its current position
+     */
+    function isDirective(node, ancestors) {
+        var parent = ancestors[ancestors.length - 1],
+            grandparent = ancestors[ancestors.length - 2];
+        return (parent.type === "Program" || parent.type === "BlockStatement" &&
+                (/Function/.test(grandparent.type))) &&
+                directives(parent).indexOf(node) >= 0;
+    }
+
+    return {
+        "ExpressionStatement": function(node) {
+
+            var type = node.expression.type,
+                ancestors = context.getAncestors();
+
+            if (
+                !/^(?:Assignment|Call|New|Update|Yield)Expression$/.test(type) &&
+                (type !== "UnaryExpression" || ["delete", "void"].indexOf(node.expression.operator) < 0) &&
+                !isDirective(node, ancestors)
+            ) {
+                context.report(node, "Expected an assignment or function call and instead saw an expression.");
+            }
+        }
+    };
+
+};