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react-native-mmkv
Fast synchronous key-value storage for React Native via react-native-mmkv (Nitro-backed). Covers creating and configuring MMKV instances, reading/writing all value types (string, number, boolean, buffer), React hooks for reactive UI, value-change listeners, encryption, AsyncStora
Overview
Fast synchronous key-value storage for React Native via react-native-mmkv (Nitro-backed). Covers creating and configuring MMKV instances, reading/writing all value types (string, number, boolean, buffer), React hooks for reactive UI, value-change listeners, encryption, AsyncStorage migration, and state-management integrations (zustand, redux-persist, jotai, react-query). Also covers storage limits, multi-process mode, and the V4 upgrade from the class-based API.
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react-native-mmkv
A focused reference for AI coding assistants working in a project that uses react-native-mmkv. Answer using the real APIs from this library — not invented ones — by routing to the matching references/*.md file below.
Mental model
react-native-mmkv is a high-performance, synchronous key-value storage library for React Native, backed by Tencent's MMKV C++ engine via Nitro Modules (JSI). It is ~30x faster than AsyncStorage.
The three things that trip people up:
- Everything is synchronous. No
await, no Promises.storage.getString('key')returns the value immediately. This is the whole point — MMKV memory-maps files and reads/writes in-process. - It's a key-value store, not a database. All data is cached in memory. Keep total size under ~100 MB; for larger datasets use react-native-nitro-sqlite or WatermelonDB. There is no fixed storage limit — but memory warnings will fire if you go too large.
- V4 is a breaking rewrite. The constructor changed from
new MMKV()tocreateMMKV(),.delete()became.remove(), andreact-native-nitro-modulesis now a required peer dependency. See the upgrade guide.
V4 requires React Native 0.75+. It works on both the New Architecture and the old architecture (Nitro is backwards compatible).
Routing table — problem to reference
Load the matching file from references/ before writing code. Each reference cites real APIs from the library.
| User is asking about… | Read |
|---|---|
Creating an MMKV instance, configuration options (id, path, encryptionKey, mode, readOnly, compareBeforeSet), the createMMKV() factory | references/create-instance.md |
Getting/setting values, all data types (string, number, boolean, buffer, objects via JSON), key management (contains, getAllKeys, remove, clearAll), size, trim, importAllFrom | references/crud-operations.md |
React hooks (useMMKVString, useMMKVNumber, useMMKVBoolean, useMMKVBuffer, useMMKVObject, useMMKV, useMMKVListener, useMMKVKeys), reactive UI | references/hooks.md |
Listening for value changes outside of React, addOnValueChangedListener, cleaning up listeners | references/listeners.md |
Encryption (encrypt, decrypt, recrypt), AES-128 vs AES-256, encryptionKey config | references/encryption.md |
| Migrating from AsyncStorage, data transfer pattern, completion flag | references/migrate-from-async-storage.md |
zustand persist middleware, redux-persist, jotai atomWithMMKV, react-query persister, MobX | references/state-management.md |
V4 upgrade, breaking changes from V3, new MMKV() → createMMKV(), .delete() → .remove() | references/upgrade-v4.md |
| Storage size limits, memory warnings, when to use a database instead | references/limits-and-gotchas.md |
If the question doesn't match any row, read references/create-instance.md first — most setup questions start there.
Installation
npm install react-native-mmkv react-native-nitro-modules
cd ios && pod install
Expo:
npx expo install react-native-mmkv react-native-nitro-modules
npx expo prebuild
After install, rebuild the app. No additional native wiring is required.
Quick reference — full MMKV instance API
import { createMMKV, existsMMKV, deleteMMKV } from 'react-native-mmkv'
const storage = createMMKV({ id: 'my-store' })
// Properties (read-only)
storage.id // string
storage.length // number of keys
storage.byteSize // total file size in bytes
storage.size // (deprecated — use byteSize)
storage.isReadOnly // boolean
storage.isEncrypted // boolean
// Write
storage.set('key', 'string' | 42 | true | arrayBuffer)
// Read
storage.getString('key') // string | undefined
storage.getNumber('key') // number | undefined
storage.getBoolean('key') // boolean | undefined
storage.getBuffer('key') // ArrayBuffer | undefined
// Keys
storage.contains('key') // boolean
storage.getAllKeys() // string[]
storage.remove('key') // boolean (NOTE: was .delete() in V3)
storage.clearAll() // void
// Encryption
storage.encrypt('password')
storage.encrypt('password', 'AES-256')
storage.decrypt()
storage.recrypt('newPassword' | undefined) // (deprecated — use encrypt()/decrypt())
// Maintenance
storage.trim() // free unused space
storage.importAllFrom(otherStorage) // returns count imported
// Listeners
const sub = storage.addOnValueChangedListener((key) => { ... })
sub.remove()
// Static helpers
existsMMKV('my-store') // boolean
deleteMMKV('my-store') // boolean
Testing
A mocked MMKV instance is automatically used when testing with Jest or Vitest. Use createMMKV() as normal in tests — no manual mocking needed.
Verifying the skill is loaded
Good test questions:
- "How do I persist zustand state with MMKV?" → should produce a
StateStorageadapter usingstorage.set,storage.getString,storage.remove— not AsyncStorage wrappers. - "How do I migrate from AsyncStorage to MMKV?" → should show the batch-copy pattern with a completion flag, not a drop-in replacement.
- "What's the storage limit?" → should explain there's no fixed limit, but data is memory-mapped so keep it under ~100 MB; use SQLite for larger datasets.
References
| File | Description |
|---|---|
| create-instance.md | createMMKV() factory, all configuration options |
| crud-operations.md | Set/get all types, key management, objects, buffers |
| hooks.md | All React hooks for reactive MMKV usage |
| listeners.md | addOnValueChangedListener, cleanup |
| encryption.md | Encrypt/decrypt instances, AES options |
| migrate-from-async-storage.md | Full migration pattern from AsyncStorage |
| state-management.md | zustand, redux-persist, jotai, react-query, MobX |
| upgrade-v4.md | V3 → V4 breaking changes and migration |
| limits-and-gotchas.md | Storage limits, multi-process, logging, common pitfalls |
Problem → Reference Mapping
| Problem | Reference | Action |
|---|---|---|
| Don't know where to start | create-instance.md | Create instance with createMMKV() |
| Need reactive UI updates | hooks.md | Use useMMKVString / useMMKVObject etc. |
| Encrypting sensitive data | encryption.md | Call storage.encrypt(key) |
| App uses AsyncStorage, want to switch | migrate-from-async-storage.md | Batch migrate then swap calls |
| Zustand/Redux/Jotai persistence | state-management.md | Wire adapter for your state lib |
| Upgrading from V3 to V4 | upgrade-v4.md | new MMKV() → createMMKV(), .delete() → .remove() |
| Storage growing too large | limits-and-gotchas.md | Trim or move to SQLite |
| Multi-process data sharing | limits-and-gotchas.md | Set mode: 'multi-process' |
File metadata
name: react-native-mmkv description: Fast synchronous key-value storage for React Native via react-native-mmkv (Nitro-backed). Covers creating and configuring MMKV instances, reading/writing all value types (string, number, boolean, buffer), React hooks for reactive UI, value-change listeners, encryption, AsyncStorage migration, and state-management integrations (zustand, redux-persist, jotai, react-query). Also covers storage limits, multi-process mode, and the V4 upgrade from the class-based API. license: MIT metadata: author: margelo scope: react-native-mmkv tags: react-native, mmkv, storage, key-value, encryption, hooks, zustand, redux, jotai, react-query, async-storage, migration, nitro-modules
View original text
---
name: react-native-mmkv
description: Fast synchronous key-value storage for React Native via react-native-mmkv (Nitro-backed). Covers creating and configuring MMKV instances, reading/writing all value types (string, number, boolean, buffer), React hooks for reactive UI, value-change listeners, encryption, AsyncStorage migration, and state-management integrations (zustand, redux-persist, jotai, react-query). Also covers storage limits, multi-process mode, and the V4 upgrade from the class-based API.
license: MIT
metadata:
author: margelo
scope: react-native-mmkv
tags: react-native, mmkv, storage, key-value, encryption, hooks, zustand, redux, jotai, react-query, async-storage, migration, nitro-modules
---
# react-native-mmkv
A focused reference for AI coding assistants working in a project that uses `react-native-mmkv`. Answer using the real APIs from this library — not invented ones — by routing to the matching `references/*.md` file below.
## Mental model
`react-native-mmkv` is a high-performance, synchronous key-value storage library for React Native, backed by Tencent's [MMKV](https://github.com/Tencent/MMKV) C++ engine via Nitro Modules (JSI). It is ~30x faster than AsyncStorage.
The three things that trip people up:
1. **Everything is synchronous.** No `await`, no Promises. `storage.getString('key')` returns the value immediately. This is the whole point — MMKV memory-maps files and reads/writes in-process.
2. **It's a key-value store, not a database.** All data is cached in memory. Keep total size under ~100 MB; for larger datasets use [react-native-nitro-sqlite](https://github.com/margelo/react-native-nitro-sqlite) or WatermelonDB. There is no fixed storage limit — but memory warnings will fire if you go too large.
3. **V4 is a breaking rewrite.** The constructor changed from `new MMKV()` to `createMMKV()`, `.delete()` became `.remove()`, and `react-native-nitro-modules` is now a required peer dependency. See the [upgrade guide](./references/upgrade-v4.md).
> **V4 requires React Native 0.75+.** It works on both the New Architecture and the old architecture (Nitro is backwards compatible).
## Routing table — problem to reference
Load the matching file from `references/` before writing code. Each reference cites real APIs from the library.
| User is asking about… | Read |
|---|---|
| Creating an MMKV instance, configuration options (`id`, `path`, `encryptionKey`, `mode`, `readOnly`, `compareBeforeSet`), the `createMMKV()` factory | [`references/create-instance.md`](./references/create-instance.md) |
| Getting/setting values, all data types (string, number, boolean, buffer, objects via JSON), key management (`contains`, `getAllKeys`, `remove`, `clearAll`), `size`, `trim`, `importAllFrom` | [`references/crud-operations.md`](./references/crud-operations.md) |
| React hooks (`useMMKVString`, `useMMKVNumber`, `useMMKVBoolean`, `useMMKVBuffer`, `useMMKVObject`, `useMMKV`, `useMMKVListener`, `useMMKVKeys`), reactive UI | [`references/hooks.md`](./references/hooks.md) |
| Listening for value changes outside of React, `addOnValueChangedListener`, cleaning up listeners | [`references/listeners.md`](./references/listeners.md) |
| Encryption (`encrypt`, `decrypt`, `recrypt`), AES-128 vs AES-256, `encryptionKey` config | [`references/encryption.md`](./references/encryption.md) |
| Migrating from AsyncStorage, data transfer pattern, completion flag | [`references/migrate-from-async-storage.md`](./references/migrate-from-async-storage.md) |
| zustand persist middleware, redux-persist, jotai `atomWithMMKV`, react-query persister, MobX | [`references/state-management.md`](./references/state-management.md) |
| V4 upgrade, breaking changes from V3, `new MMKV()` → `createMMKV()`, `.delete()` → `.remove()` | [`references/upgrade-v4.md`](./references/upgrade-v4.md) |
| Storage size limits, memory warnings, when to use a database instead | [`references/limits-and-gotchas.md`](./references/limits-and-gotchas.md) |
If the question doesn't match any row, read [`references/create-instance.md`](./references/create-instance.md) first — most setup questions start there.
## Installation
```bash
npm install react-native-mmkv react-native-nitro-modules
cd ios && pod install
```
Expo:
```bash
npx expo install react-native-mmkv react-native-nitro-modules
npx expo prebuild
```
After install, rebuild the app. No additional native wiring is required.
## Quick reference — full MMKV instance API
```ts
import { createMMKV, existsMMKV, deleteMMKV } from 'react-native-mmkv'
const storage = createMMKV({ id: 'my-store' })
// Properties (read-only)
storage.id // string
storage.length // number of keys
storage.byteSize // total file size in bytes
storage.size // (deprecated — use byteSize)
storage.isReadOnly // boolean
storage.isEncrypted // boolean
// Write
storage.set('key', 'string' | 42 | true | arrayBuffer)
// Read
storage.getString('key') // string | undefined
storage.getNumber('key') // number | undefined
storage.getBoolean('key') // boolean | undefined
storage.getBuffer('key') // ArrayBuffer | undefined
// Keys
storage.contains('key') // boolean
storage.getAllKeys() // string[]
storage.remove('key') // boolean (NOTE: was .delete() in V3)
storage.clearAll() // void
// Encryption
storage.encrypt('password')
storage.encrypt('password', 'AES-256')
storage.decrypt()
storage.recrypt('newPassword' | undefined) // (deprecated — use encrypt()/decrypt())
// Maintenance
storage.trim() // free unused space
storage.importAllFrom(otherStorage) // returns count imported
// Listeners
const sub = storage.addOnValueChangedListener((key) => { ... })
sub.remove()
// Static helpers
existsMMKV('my-store') // boolean
deleteMMKV('my-store') // boolean
```
## Testing
A mocked MMKV instance is automatically used when testing with Jest or Vitest. Use `createMMKV()` as normal in tests — no manual mocking needed.
## Verifying the skill is loaded
Good test questions:
- *"How do I persist zustand state with MMKV?"* → should produce a `StateStorage` adapter using `storage.set`, `storage.getString`, `storage.remove` — not AsyncStorage wrappers.
- *"How do I migrate from AsyncStorage to MMKV?"* → should show the batch-copy pattern with a completion flag, not a drop-in replacement.
- *"What's the storage limit?"* → should explain there's no fixed limit, but data is memory-mapped so keep it under ~100 MB; use SQLite for larger datasets.
## References
| File | Description |
|------|-------------|
| [create-instance.md](./references/create-instance.md) | `createMMKV()` factory, all configuration options |
| [crud-operations.md](./references/crud-operations.md) | Set/get all types, key management, objects, buffers |
| [hooks.md](./references/hooks.md) | All React hooks for reactive MMKV usage |
| [listeners.md](./references/listeners.md) | `addOnValueChangedListener`, cleanup |
| [encryption.md](./references/encryption.md) | Encrypt/decrypt instances, AES options |
| [migrate-from-async-storage.md](./references/migrate-from-async-storage.md) | Full migration pattern from AsyncStorage |
| [state-management.md](./references/state-management.md) | zustand, redux-persist, jotai, react-query, MobX |
| [upgrade-v4.md](./references/upgrade-v4.md) | V3 → V4 breaking changes and migration |
| [limits-and-gotchas.md](./references/limits-and-gotchas.md) | Storage limits, multi-process, logging, common pitfalls |
## Problem → Reference Mapping
| Problem | Reference | Action |
|---------|-----------|--------|
| Don't know where to start | [create-instance.md](./references/create-instance.md) | Create instance with `createMMKV()` |
| Need reactive UI updates | [hooks.md](./references/hooks.md) | Use `useMMKVString` / `useMMKVObject` etc. |
| Encrypting sensitive data | [encryption.md](./references/encryption.md) | Call `storage.encrypt(key)` |
| App uses AsyncStorage, want to switch | [migrate-from-async-storage.md](./references/migrate-from-async-storage.md) | Batch migrate then swap calls |
| Zustand/Redux/Jotai persistence | [state-management.md](./references/state-management.md) | Wire adapter for your state lib |
| Upgrading from V3 to V4 | [upgrade-v4.md](./references/upgrade-v4.md) | `new MMKV()` → `createMMKV()`, `.delete()` → `.remove()` |
| Storage growing too large | [limits-and-gotchas.md](./references/limits-and-gotchas.md) | Trim or move to SQLite |
| Multi-process data sharing | [limits-and-gotchas.md](./references/limits-and-gotchas.md) | Set `mode: 'multi-process'` |
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Review before install: Avoid automatic install
License: MIT
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- Stars/forks activity: 161 stars, 7 forks; issue activity unavailable in current metadata
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Source & usage notes
Indexed
Repository metadata and review signals are advisory. Popularity, source discovery and successful execution are different facts.
- Source repository
- margelo/react-native-skills
- License
- MIT
- Version
- 1.0.0
- Last GitHub push
- Aug 21, 2026
- Registry updated
- Sep 1, 2026
- Instruction path
- skills/react-native-mmkv/SKILL.md
Version reported in registry metadata; check source releases before relying on it.
Quality
66/100
Promising
Trust
61/100
Sandbox only
Audit
74/100
Risky
- Dependency or permission surface needs review
- Permission surface may require sandboxing
- Financial research output is not financial advice; require human review before any live investment decision
- Potential broker, wallet, exchange, or real-money execution surface; sandbox and explicit approval are required
- Financial research output is not financial advice; require human review before any live investment decision.
- This skill may touch real-money trading, broker, wallet, or exchange operations; use only in a sandbox with explicit approval.
- Quality score needs review
- Permission surface needs review: secrets or environment access, shell or command execution
- Stars/forks activity: 161 stars, 7 forks; issue activity unavailable in current metadata
- Dependency/runtime risk: command execution surface, credential or environment access
- Permission surface: secrets or environment access, shell or command execution
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"trust": {
"score": 69,
"label": "Manual review",
"version": "trust-score-v4",
"install_policy": "block",
"evidence": {
"stars": "161 GitHub stars",
"repoActivity": "161 stars, 7 forks",
"lastPushed": "2mo since push",
"license": "MIT",
"repository": "https://github.com/margelo/react-native-skills/tree/main/skills/react-native-mmkv",
"install": "npx skills add margelo/react-native-skills --skill react-native-mmkv",
"installSafety": "standard package or runtime install path",
"permissionSurface": "secrets or environment access, shell or command execution",
"documentation": "Strong README/SKILL.md context",
"agentOutcomes": "No agent outcome data yet"
},
"outcome_evidence": {
"total": 0,
"successes": 0,
"failures": 0,
"not_relevant": 0,
"success_rate": null,
"recent_success_rate": null,
"recent_failure_rate": null,
"install_attempts": 0,
"install_success_rate": null,
"risk_blocked": 0,
"setup_required": 0,
"avg_output_quality": null,
"production_outcomes": 0,
"last_outcome_at": null,
"label": "No agent outcome data yet"
},
"auto_install": {
"allowed": false,
"sandbox_required": true,
"reason": "Do not auto-install. Inspect the source, dependencies, and permission surface first."
},
"best_for": [
"design-creative",
"agent-skill"
],
"known_risks": [
"Financial research output is not financial advice; require human review before any live investment decision.",
"This skill may touch real-money trading, broker, wallet, or exchange operations; use only in a sandbox with explicit approval.",
"Quality score needs review",
"Permission surface needs review: secrets or environment access, shell or command execution",
"Stars/forks activity: 161 stars, 7 forks; issue activity unavailable in current metadata",
"Dependency/runtime risk: command execution surface, credential or environment access",
"Permission surface: secrets or environment access, shell or command execution"
]
},
"agent_proven": {
"version": "agent-proven-v1",
"score": 0,
"tier": "unproven",
"label": "Needs first agent run",
"summary": "No agent outcome reports yet. Use Resolve, run one narrow sandbox task, then report the result.",
"metrics": {
"totalOutcomes": 0,
"successfulOutcomes": 0,
"failedOutcomes": 0,
"installAttempts": 0,
"installSuccessRate": null,
"successRate": null,
"recentSuccessRate": null,
"recentFailureRate": null,
"riskBlocked": 0,
"setupRequired": 0,
"notRelevant": 0,
"avgOutputQuality": null,
"avgTimeToUsefulMs": null,
"productionOutcomes": 0,
"humanReviewRequired": 0,
"uniqueAgents": 0,
"lastOutcomeAt": null
},
"signals": [],
"penalties": [
"No real agent outcome evidence yet"
]
},
"audit": {
"score": 74,
"risk_level": "risky",
"risk_label": "Risky",
"warnings": [
"Dependency or permission surface needs review",
"Permission surface may require sandboxing",
"Financial research output is not financial advice; require human review before any live investment decision",
"Potential broker, wallet, exchange, or real-money execution surface; sandbox and explicit approval are required",
"Financial research output is not financial advice; require human review before any live investment decision.",
"This skill may touch real-money trading, broker, wallet, or exchange operations; use only in a sandbox with explicit approval.",
"Quality score needs review",
"Permission surface needs review: secrets or environment access, shell or command execution"
]
},
"safety_gate": {
"tier": "blocked",
"label": "Blocked for auto-install",
"auto_install_policy": "block",
"auto_install_allowed": false,
"human_review_required": true,
"blocked": true,
"recommended_action": "Do not auto-install. Inspect the source, dependencies, and permission surface first."
},
"quality": {
"score": 66,
"label": "Promising"
},
"supply": {
"track": "Design and creative production",
"scenario": "Design and creative",
"maintenance": "2mo since push",
"risk": "Risky"
},
"alternative_skills": [],
"do_not_use_when": [
"teams that need a vendor-supported SLA",
"high-compliance environments without internal security review",
"No major risk signals from current metadata",
"Audit risk risky exceeds max_risk=medium",
"High-risk permission hints: Shell or command execution, Secrets or environment access",
"Dependency or permission surface needs review",
"Permission surface may require sandboxing",
"Financial research output is not financial advice; require human review before any live investment decision"
],
"agent_contract": {
"task_input": "Use react-native-mmkv in an agent workflow",
"recommended_action": "Do not auto-install. Inspect the source, dependencies, and permission surface first.",
"install_policy": "block",
"minimum_review_before_use": [
"Trust: 69/100 Manual review",
"Audit: 74/100 Risky",
"Safety: 30/100 Avoid automatic install",
"Review repository, license, install command, and permission surface before production use."
],
"expected_agent_output": {
"selected_skill": "margelo-react-native-mmkv (react-native-mmkv)",
"install_command": "npx skills add margelo/react-native-skills --skill react-native-mmkv",
"risk_summary": "Risky; Blocked for auto-install; Review before production",
"verification_result": "Report the smallest successful task, files touched, warnings, and any missing setup."
}
},
"outcome_feedback": {
"endpoint": "https://www.openagentskill.com/api/agent/outcome",
"method": "POST",
"requires_resolve_event_id": true,
"event_id_source": "Use install_receipt.outcome_feedback.event_id or feedback.event_id returned by /api/agent/resolve for the current task.",
"expected_outcomes": [
"success",
"failed",
"not_relevant",
"blocked_by_risk",
"setup_required"
],
"payload_template": {
"event_id": "<install_receipt.outcome_feedback.event_id or feedback.event_id from /api/agent/resolve>",
"skill_slug": "margelo-react-native-mmkv",
"task": "Use react-native-mmkv in an agent workflow",
"agent": "codex",
"outcome": "success",
"install_used": true,
"risk_blocked": false,
"setup_required": false,
"task_success": true,
"output_quality": 4,
"error_type": null,
"human_review_required": false,
"workspace": "sandbox",
"time_to_useful_ms": 120000,
"notes": "Report the smallest successful task, setup friction, files touched, and risk notes."
}
},
"endpoints": {
"web": "https://www.openagentskill.com/skills/margelo-react-native-mmkv",
"api": "https://www.openagentskill.com/api/agent/skills/margelo-react-native-mmkv",
"audit": "https://www.openagentskill.com/skills/margelo-react-native-mmkv/audit",
"eval": "https://www.openagentskill.com/api/agent/evals?slug=margelo-react-native-mmkv&task=Use%20react-native-mmkv%20in%20an%20agent%20workflow&max_risk=medium",
"resolve": "https://www.openagentskill.com/api/agent/resolve?task=Use%20react-native-mmkv%20in%20an%20agent%20workflow&agent=codex&max_risk=medium",
"receipt": "https://www.openagentskill.com/api/agent/receipt?task=Use%20react-native-mmkv%20in%20an%20agent%20workflow&agent=codex&max_risk=medium&format=text",
"install": "https://www.openagentskill.com/api/skills/margelo-react-native-mmkv/install",
"manifest": "https://www.openagentskill.com/api/registry/manifest/margelo-react-native-mmkv"
}
}For the creator
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