diff --git a/src/vanilla/utils/proxyMap.ts b/src/vanilla/utils/proxyMap.ts
index 12219f00..6bcc24e3 100644
--- a/src/vanilla/utils/proxyMap.ts
+++ b/src/vanilla/utils/proxyMap.ts
@@ -178,12 +178,36 @@ export function proxyMap<K, V>(entries?: Iterable<[K, V]> | undefined | null) {
     toJSON(): Map<K, V> {
       return new Map(this.entries())
     },
-    // [ONLY-TS-5.9.3] [ONLY-TS-5.8.3] [ONLY-TS-5.7.3] [ONLY-TS-5.6.3] @ts-expect-error ignore
-    getOrInsert() {
-      throw new Error('not implemented')
+    // [ONLY-TS-5.9.3] [ONLY-TS-5.8.3] [ONLY-TS-5.7.3] [ONLY-TS-5.6.3] @ts-ignore ignore
+    getOrInsert(key: K, defaultValue: V): V {
+      if (!isProxy(this)) {
+        throw new Error('Cannot perform mutations on a snapshot')
+      }
+      const index = indexMap.get(key)
+      if (index !== undefined) {
+        return this.data[index]!
+      }
+      indexMap.set(key, this.index)
+      this.data[this.index] = defaultValue
+      this.index++
+      this.epoch++
+      return defaultValue
     },
-    getOrInsertComputed() {
-      throw new Error('not implemented')
+    // [ONLY-TS-5.9.3] [ONLY-TS-5.8.3] [ONLY-TS-5.7.3] [ONLY-TS-5.6.3] @ts-ignore ignore
+    getOrInsertComputed(key: K, callbackFn: (key: K) => V): V {
+      if (!isProxy(this)) {
+        throw new Error('Cannot perform mutations on a snapshot')
+      }
+      const index = indexMap.get(key)
+      if (index !== undefined) {
+        return this.data[index]!
+      }
+      const value = callbackFn(key)
+      indexMap.set(key, this.index)
+      this.data[this.index] = value
+      this.index++
+      this.epoch++
+      return value
     },
   }