1 files changed, 95 insertions, 0 deletions
diff --git a/tools/eslint/lib/rules/no-fallthrough.js b/tools/eslint/lib/rules/no-fallthrough.js
new file mode 100644
index 0000000000..a137fb3f80
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+++ b/tools/eslint/lib/rules/no-fallthrough.js
@@ -0,0 +1,95 @@
+/**
+ * @fileoverview Rule to flag fall-through cases in switch statements.
+ * @author Matt DuVall <http://mattduvall.com/>
+ */
+"use strict";
+
+
+var FALLTHROUGH_COMMENT = /falls\sthrough/;
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = function(context) {
+
+    var switches = [];
+
+    return {
+
+        "SwitchCase": function(node) {
+
+            var consequent = node.consequent,
+                switchData = switches[switches.length - 1],
+                i,
+                comments,
+                comment;
+
+            /*
+             * Some developers wrap case bodies in blocks, so if there is just one
+             * node and it's a block statement, check inside.
+             */
+            if (consequent.length === 1 && consequent[0].type === "BlockStatement") {
+                consequent = consequent[0];
+            }
+
+            // checking on previous case
+            if (!switchData.lastCaseClosed) {
+
+                // a fall through comment will be the last trailing comment of the last case
+                comments = context.getComments(switchData.lastCase).trailing;
+                comment = comments[comments.length - 1];
+
+                // unless the user doesn't like semicolons, in which case it's first leading comment of this case
+                if (!comment) {
+                    comments = context.getComments(node).leading;
+                    comment = comments[comments.length - 1];
+                }
+
+                // check for comment
+                if (!comment || !FALLTHROUGH_COMMENT.test(comment.value)) {
+
+                    context.report(switchData.lastCase,
+                        "Expected a \"break\" statement before \"{{code}}\".",
+                        { code: node.test ? "case" : "default" });
+                }
+            }
+
+            // now dealing with the current case
+            switchData.lastCaseClosed = false;
+            switchData.lastCase = node;
+
+            // try to verify using statements - go backwards as a fast path for the search
+            if (consequent.length) {
+                for (i = consequent.length - 1; i >= 0; i--) {
+                    if (/(?:Break|Continue|Return|Throw)Statement/.test(consequent[i].type)) {
+                        switchData.lastCaseClosed = true;
+                        break;
+                    }
+                }
+            } else {
+                // the case statement has no statements, so it must logically fall through
+                switchData.lastCaseClosed = true;
+            }
+
+            /*
+             * Any warnings are triggered when the next SwitchCase occurs.
+             * There is no need to warn on the last SwitchCase, since it can't
+             * fall through to anything.
+             */
+        },
+
+        "SwitchStatement": function(node) {
+            switches.push({
+                node: node,
+                lastCaseClosed: true,
+                lastCase: null
+            });
+        },
+
+        "SwitchStatement:exit": function() {
+            switches.pop();
+        }
+    };
+
+};