fix(render-props): keep render prop state alive across renders

src/backfills.ts

@@ -1,3 +1,5 @@
+import { isObject } from './is-object.js';
+
 const DATA_CONTEXT_PROP = 'dataContext';
 let once = false;
 
@@ -6,6 +8,11 @@ let once = false;
  * the `dataContext` prop returning the original data
  */
 export function withDataContext(data: unknown) {
+  // Proxy throws on primitives.
+  if (!isObject(data)) {
+    return data;
+  }
+
   return new Proxy(data as any, {
     get(target, prop, receiver) {
       if (prop === DATA_CONTEXT_PROP) {

src/equal.ts

@@ -1,91 +1,115 @@
-export function equal<T>(a: unknown, b: T, visited = new WeakSet()): boolean {
+import { isObject } from './is-object.js';
+
+type Visited = WeakMap<object, WeakSet<object>>;
+
+export function equal<T>(a: unknown, b: T, visited?: Visited): boolean {
   // Early return
-  if (Object.is(a, b)) return true;
+  if (Object.is(a, b)) {
+    return true;
+  }
 
-  if (isObject(a) && isObject(b)) {
-    if (a.constructor !== b.constructor) return false;
+  if (!isObject(a) || !isObject(b)) {
+    return false;
+  }
 
-    // Circular references
-    if (visited.has(a) && visited.has(b)) {
-      return true;
-    }
+  if (a.constructor !== b.constructor) {
+    return false;
+  }
 
-    visited.add(a);
-    visited.add(b);
-
-    // RegExp
-    if (isRegExp(a) && isRegExp(b)) return a.source === b.source && a.flags === b.flags;
-
-    // Maps
-    if (a instanceof Map && b instanceof Map) {
-      if (a.size !== b.size) return false;
-      for (const [keyA, valueA] of a.entries()) {
-        let found = false;
-        for (const [keyB, valueB] of b.entries()) {
-          if (equal(keyA, keyB, visited) && equal(valueA, valueB, visited)) {
-            found = true;
-            break;
-          }
-        }
-        if (!found) return false;
-      }
-      return true;
-    }
+  // Track pairs, not objects: failed Set/Map probes must not count as visited.
+  const pairs: Visited = visited ?? new WeakMap();
+  const pending = pairs.get(a) ?? new WeakSet<object>();
 
-    // Sets
-    if (a instanceof Set && b instanceof Set) {
-      if (a.size !== b.size) return false;
-      for (const valueA of a) {
-        let found = false;
-        for (const valueB of b) {
-          if (equal(valueA, valueB, visited)) {
-            found = true;
-            break;
-          }
-        }
-        if (!found) return false;
-      }
-      return true;
+  if (pending.has(b)) {
+    return true;
+  }
+
+  pairs.set(a, pending.add(b));
+
+  try {
+    return compare(a, b, pairs);
+  } finally {
+    pending.delete(b);
+  }
+}
+
+function compare(a: object, b: object, visited: Visited): boolean {
+  // RegExp
+  if (isRegExp(a) && isRegExp(b)) {
+    return a.source === b.source && a.flags === b.flags;
+  }
+
+  // Map entries iterate as [key, value] arrays.
+  if ((a instanceof Map && b instanceof Map) || (a instanceof Set && b instanceof Set)) {
+    return a.size === b.size && matchOneToOne(a, b, visited);
+  }
+
+  // Arrays
+  if (Array.isArray(a) && Array.isArray(b)) {
+    const length = a.length;
+
+    if (length !== b.length) {
+      return false;
     }
 
-    // Arrays
-    if (Array.isArray(a) && Array.isArray(b)) {
-      const length = a.length;
-      if (length !== b.length) return false;
-      for (let i = 0; i < length; i++) {
-        if (!equal(a[i], b[i], visited)) return false;
+    for (let i = 0; i < length; i++) {
+      if (!equal(a[i], b[i], visited)) {
+        return false;
       }
-      return true;
     }
+    return true;
+  }
 
-    // toPrimitive
-    if (a.valueOf !== Object.prototype.valueOf) return a.valueOf() === b.valueOf();
-    // Strings based
-    if (a.toString !== Object.prototype.toString) return a.toString() === b.toString();
+  // toPrimitive; absent on null-prototype objects.
+  if (typeof a.valueOf === 'function' && a.valueOf !== Object.prototype.valueOf) {
+    return a.valueOf() === b.valueOf();
+  }
 
-    const aKeys = Object.keys(a);
-    const bKeys = Object.keys(b);
-    if (aKeys.length !== bKeys.length) return false;
+  // Strings based
+  if (typeof a.toString === 'function' && a.toString !== Object.prototype.toString) {
+    return a.toString() === b.toString();
+  }
 
-    for (const key of aKeys) {
-      if (!Object.hasOwn(b, key)) return false;
-    }
+  const aKeys = Object.keys(a);
+  const bKeys = Object.keys(b);
 
-    for (const key of aKeys) {
-      if (!equal(a[key as keyof typeof a], b[key as keyof typeof b], visited)) return false;
-    }
+  if (aKeys.length !== bKeys.length) {
+    return false;
+  }
 
-    visited.delete(a);
-    visited.delete(b);
+  for (const key of aKeys) {
+    if (!Object.hasOwn(b, key)) {
+      return false;
+    }
+  }
 
-    return true;
+  for (const key of aKeys) {
+    if (!equal(a[key as keyof typeof a], b[key as keyof typeof b], visited)) {
+      return false;
+    }
   }
 
-  return false;
+  return true;
 }
 
-function isObject(value: unknown): value is object {
-  return value != null && typeof value === 'object';
+/** Greedy suffices: `equal` is transitive. */
+function matchOneToOne(
+  left: Iterable<unknown>,
+  right: Iterable<unknown>,
+  visited: Visited,
+): boolean {
+  const pool = [...right];
+
+  for (const a of left) {
+    const index = pool.findIndex((b) => equal(a, b, visited));
+
+    if (index < 0) {
+      return false;
+    }
+    pool.splice(index, 1);
+  }
+
+  return true;
 }
 
 function isRegExp(value: unknown): value is RegExp {

src/is-object.ts

@@ -0,0 +1,3 @@
+export function isObject(value: unknown): value is Record<string, unknown> {
+  return value !== null && typeof value === 'object';
+}