Fix hashing by generating random, stable key for non-objects

Any object without any enumerable keys that isn't a plain
object will now receive a random key.

packages/core/src/utils/stringifyVariables.test.ts

@@ -34,14 +34,17 @@ it('stringifies dates correctly', () => {
   expect(stringifyVariables(date)).toBe(date.toJSON());
 });
 
+it('stringifies dictionaries (Object.create(null)) correctly', () => {
+  expect(stringifyVariables(Object.create(null))).toBe('{}');
+});
+
 it('stringifies files correctly', () => {
   const file = new File([0] as any, 'test.js');
   Object.defineProperty(file, 'lastModified', { value: 123 });
+  const str = stringifyVariables(file);
+  expect(str).toBe(stringifyVariables(file));
 
-  expect(stringifyVariables(file)).toBe(
-    stringifyVariables({
-      name: 'test.js',
-      lastModified: 123,
-    })
-  );
+  const otherFile = new File([0] as any, 'otherFile.js');
+  Object.defineProperty(otherFile, 'lastModified', { value: 234 });
+  expect(str).not.toBe(stringifyVariables(otherFile));
 });

packages/core/src/utils/stringifyVariables.ts

@@ -1,4 +1,5 @@
 const seen = new Set();
+const cache = new WeakMap();
 
 const stringify = (x: any): string => {
   if (x === undefined) {
@@ -26,11 +27,18 @@ const stringify = (x: any): string => {
     return out;
   } else if (seen.has(x)) {
     throw new TypeError('Converting circular structure to JSON');
-  } else if (typeof File === 'function' && x instanceof File) {
-    return stringify({ name: x.name, lastModified: x.lastModified });
   }
 
   const keys = Object.keys(x).sort();
+  if (!keys.length && x.constructor && x.constructor !== Object) {
+    const key =
+      cache.get(x) ||
+      Math.random()
+        .toString(36)
+        .slice(2);
+    cache.set(x, key);
+    return `{"__key":"${key}"}`;
+  }
 
   seen.add(x);
   out = '{';