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node_modules/fast-equals/recipes/legacy-circular-equal-support.md

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+# Legacy environment support for circular equal comparators
+
+Starting in `4.x.x`, `WeakMap` is expected to be available in the environment. All modern browsers support this global object, however there may be situations where a legacy environmental support is required (example: IE11). If you need to support such an environment and polyfilling is not an option, creating a custom comparator that uses a custom cache implementation with the same contract is a simple solution.
+
+```ts
+import { createCustomEqual, sameValueZeroEqual } from 'fast-equals';
+import type { Cache } from 'fast-equals';
+
+function getCache(): Cache<any, any> {
+  const entries: Array<[object, any]> = [];
+
+  return {
+    delete(key) {
+      for (let index = 0; index < entries.length; ++index) {
+        if (entries[index][0] === key) {
+          entries.splice(index, 1);
+          return true;
+        }
+      }
+
+      return false;
+    },
+
+    get(key) {
+      for (let index = 0; index < entries.length; ++index) {
+        if (entries[index][0] === key) {
+          return entries[index][1];
+        }
+      }
+    },
+
+    set(key, value) {
+      for (let index = 0; index < entries.length; ++index) {
+        if (entries[index][0] === key) {
+          entries[index][1] = value;
+          return this;
+        }
+      }
+
+      entries.push([key, value]);
+
+      return this;
+    },
+  };
+}
+
+interface Meta {
+  customMethod(): void;
+  customValue: string;
+}
+
+const meta = {
+  customMethod() {
+    console.log('hello!');
+  },
+  customValue: 'goodbye',
+};
+
+const circularDeepEqual = createCustomEqual<Cache>({
+  circular: true,
+  createState: () => ({
+    cache: getCache(),
+    meta,
+  }),
+});
+
+const circularShallowEqual = createCustomEqual<Cache>({
+  circular: true,
+  comparator: sameValueZeroEqual,
+  createState: () => ({
+    cache: getCache(),
+    meta,
+  }),
+});
+```