verify traversal visitors

src/babel/messages.js

@@ -22,7 +22,11 @@ export const MESSAGES = {
   codeGeneratorDeopt: "Note: The code generator has deoptimised the styling of $1 as it exceeds the max of $2.",
   missingTemplatesDirectory: "no templates directory - this is most likely the result of a broken `npm publish`. Please report to https://github.com/babel/babel/issues",
   unsupportedOutputType: "Unsupported output type $1",
-  illegalMethodName: "Illegal method name $1"
+  illegalMethodName: "Illegal method name $1",
+  traverseNeedsParent: "Must pass a scope and parentPath unless traversing a Program/File got a $1 node",
+  traverseVerifyRootFunction: "You passed `traverse()` a function when it expected a visitor object, are you sure you didn't mean `{ enter: Function }`?",
+  traverseVerifyVisitorFunction: "Hey! You passed \`traverse()\` a visitor object with the key $1 that's a straight up `Function` instead of `{ enter: Function }`. You need to normalise it with `traverse.explode(visitor)`.",
+  traverseVerifyVisitorProperty: "You passed `traverse()` a visitor object with the property $1 that has the invalid property $2"
 };
 
 export function get(key: String, ...args) {

src/babel/traversal/index.js

@@ -1,4 +1,5 @@
 import TraversalContext from "./context";
+import * as messages from "../messages";
 import includes from "lodash/collection/includes";
 import * as t from "../types";
 
@@ -7,7 +8,7 @@ export default function traverse(parent, opts, scope, state, parentPath) {
 
   if (!opts.noScope && !scope) {
     if (parent.type !== "Program" && parent.type !== "File") {
-      throw new Error(`Must pass a scope and parentPath unless traversing a Program/File got a ${parent.type} node`);
+      throw new Error(messages.get("traverseNeedsParent", parent.type));
     }
   }
 
@@ -15,6 +16,8 @@ export default function traverse(parent, opts, scope, state, parentPath) {
   if (!opts.enter) opts.enter = function () { };
   if (!opts.exit) opts.exit = function () { };
 
+  traverse.verify(opts);
+
   // array of nodes
   if (Array.isArray(parent)) {
     for (var i = 0; i < parent.length; i++) {
@@ -25,6 +28,39 @@ export default function traverse(parent, opts, scope, state, parentPath) {
   }
 }
 
+/**
+ * Quickly iterate over some traversal options and validate it.
+ */
+
+traverse.verify = function (opts) {
+  if (opts._verified) return;
+
+  if (typeof opts === "function") {
+    throw new Error(messages.get("traverseVerifyRootFunction"));
+  }
+
+  for (var key in opts) {
+    // it's all good
+    if (key === "blacklist") continue;
+
+    var opt = opts[key];
+
+    if (typeof opt === "function") {
+      // it's all good, it's fine for this key to be a function
+      if (key === "enter" || key === "exit") continue;
+
+      throw new Error(messages.get("traverseVerifyVisitorFunction", key));
+    } else if (typeof opt === "object") {
+      for (var key2 in opt) {
+        if (key2 === "enter" || key2 === "exit") continue;
+        throw new Error(messages.get("traverseVerifyVisitorProperty", key, key2));
+      }
+    }
+  }
+
+  opts._verified = true;
+};
+
 traverse.node = function (node, opts, scope, state, parentPath) {
   var keys = t.VISITOR_KEYS[node.type];
   if (!keys) return;