just use a Set

lib/resolve-external.js

@@ -44,7 +44,7 @@ function resolveExternal (parser, options) {
  * @param {string} path - The full path of `obj`, possibly with a JSON Pointer in the hash
  * @param {$Refs} $refs
  * @param {$RefParserOptions} options
- * @param {WeakMap} seen - Internal.
+ * @param {Set} seen - Internal.
  *
  * @returns {Promise[]}
  * Returns an array of promises. There will be one promise for each JSON reference in `obj`.
@@ -53,11 +53,11 @@ function resolveExternal (parser, options) {
  * then the corresponding promise will internally reference an array of promises.
  */
 function crawl (obj, path, $refs, options, seen) {
-  seen = seen || new WeakMap();
+  seen = seen || new Set();
   let promises = [];
 
   if (obj && typeof obj === "object" && !ArrayBuffer.isView(obj) && !seen.has(obj)) {
-    seen.set(obj, 1); // Track previously seen objects to avoid infinite recursion
+    seen.add(obj); // Track previously seen objects to avoid infinite recursion
     if ($Ref.isExternal$Ref(obj)) {
       promises.push(resolve$Ref(obj, path, $refs, options));
     }