MMKV does not persist data.

[CD-Z]

First of all I already saw #1011 but it couldn't help me.

Recently I tried upgrading a bare app with epxo modules to expo 55 and rn 83 from 54 and 81. However after getting the app running again mmkv doesn't persist the data. I was now debugging this problem for three days but still have no clue on solving this.

I also tried creating a smaller example app, but no matter what i did it was always working.

I tried:

• Removing custom plugins
• Removing the appIdSuffix
• Upgrading to the latest mmkv version
• removing the scr folder
• copying the android folder in a new app
• build the project on a different pc
• build it on an emulator
• build it on an actual phone
• ...

But still the new app was always working and the original wasn't.
I don't know if I have hit some edgecase or just do something wrong, but I am out of ideas and hope someone here can help me.

Also what I noticed is that mmkv doesn't create a new folder or read the existing one. Other packages like op-sqlite continue working normally, even though they write in the same folder.

Here is the PR to upgrade the app: CD-Z/lnreader#28

[maintenance-hans]

Guten Tag, Hans here.

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[WeberG619]

Hi,

I see you've spent three days debugging MMKV persistence after upgrading to Expo 55 / RN 0.83 — and frustratingly, it works fine in a fresh app but not in the original. The fact that op-sqlite continues working in the same directory while MMKV doesn't even create its folder suggests something in your native build config or module resolution changed during the upgrade.

I'm a full-stack developer working across Python, C#, and TypeScript daily — building everything from REST APIs and database systems to desktop applications and automation pipelines.

What I deliver:

• Clean, tested code with proactive communication — no surprises
• Experience with JavaScript and React Native debugging in production environments
• Fast turnaround — I typically have a working first version within days, not weeks

For this issue:
I'd start by diffing your android/build.gradle, gradle.properties, and AndroidManifest.xml against the working sample app to isolate what's breaking MMKV's native module initialization. Then trace the actual file I/O calls to see where MMKV thinks it should write versus where your app's sandboxed storage lives.

Can start immediately. Have you checked whether MMKV's native module is actually loading, or does MMKV.initialize() silently fail?

Weber Gouin

[devappsstudio]

Hi,

I encountered the issue ‘MMKV does not persist data’ after I upgraded my Expo project to Expo 55:
"expo": "55.0.5",
"react": "19.2.0",
"react-native": "0.83.2",
"react-native-mmkv": "4.3.0",
"react-native-nitro-modules": "0.35.2",
... ,

and here is my solution that i use like react-native-mmkv+4.3.0.patch:

index 9342961..751b248 100644
--- a/node_modules/react-native-mmkv/src/isTest.ts
+++ b/node_modules/react-native-mmkv/src/isTest.ts
@@ -4,6 +4,7 @@ export function isTest(): boolean {
     return false
   }
   return (
-    process.env.JEST_WORKER_ID != null || process.env.VITEST_WORKER_ID != null
+    (process.env.JEST_WORKER_ID != null || process.env.VITEST_WORKER_ID != null) &&
+    process.env.NODE_ENV === 'test'
   )
 }

this helped, but more precise testing is needed in different cases, and it would be great to hear thought on this from @mrousavy , thanks in advance, and I will be glad if this helps someone.

[riteshshukla04]

Hey @CD-Z , Thanks for the issue. The issue is not in react-native-mmkv but in how ENVs are read .

react-native-dotenv is setting JEST_WORKER_ID to be a truthy value which makes createMMKV to run the Mock env .
For quick fix you can patch dot env to something like below .

--- a/node_modules/react-native-dotenv/index.js                                                             
  +++ b/node_modules/react-native-dotenv/index.js
  @@ -152,7 +152,9 @@ module.exports = (api, options) => {                                                    
             const key = path.toComputedKey()                                                               
             if (t.isStringLiteral(key)) {                                                                  
               const importedId = key.value
  -            const value = (env && importedId in env) ? env[importedId] : process.env[importedId]
  +            // FIX: Only inline values from .env files, never fall back to build-time process.env
  +            // (build-time process.env leaks JEST_WORKER_ID from Metro's jest-worker)
  +            const value = (env && importedId in env) ? env[importedId] : undefined
               if (value !== undefined) {
                 path.replaceWith(t.valueToNode(value))
               }

[CD-Z]

Hey @riteshshukla04, thank you very much for your help. I didn't even think it could be an issue with env variables. I changed from react-native-dotenv to react-native-config as dotenv hasn't been updated in 2 years and I don't want to deal with patch files. Now it is working perfectly fine again.

Maybe react-native-mmkv should change the isTest method to something more robust like @devappsstudio suggested or add a warning log that it is now in test mode. For this reason I will keep the issue open even though my problem is solved.

[mrousavy]

Maybe react-native-mmkv should change the isTest method

react-native-mmkv is open source

[CD-Z]

I know and I didn't mean to demand anything but in my opinion this is something that should be decided by someone who is used to this project and knows how to solve this situation without creating new problems.

[mrousavy]

Makes sense!

After chatting about this a bit I decided to close this issue - this is not related to react-native-mmkv and should probably be fixed in the dotenv package

[kKaskak]

@mrousavy i had the exact same issue with the storage not being persisted after upgrading the expo version, mmkv pkg version did not change, i migrated from the react-native-dotenv to expo native env solution with EXPO_PUBLIC prefix
ref doc: https://docs.expo.dev/guides/environment-variables/

and the issue persisted i had to make the same patch as @riteshshukla04 and add a patch script

[mrousavy]

Can you reproduce this via a Harness test in this repo?

[kKaskak]

Can you reproduce this via a Harness test in this repo?

investigated and i could not reproduce, i rebuild everything again and the patch was not necessary anymore, probably cache issue

thank you all works well now without the react-native-dotenv package

[riteshshukla04]

I just want to point out that that the fix
(process.env.JEST_WORKER_ID != null || process.env.VITEST_WORKER_ID != null) && process.env.NODE_ENV === 'test' is not correct . Its sad that LLM generates this.

This is how expo determines the testing env .
https://github.com/expo/expo/blob/31d4a6cc20eb3c1c6c015ed30b02b9b1b3119769/packages/expo-notifications/src/index.ts#L3-L5
NODE_ENV may or may not be test in JEST

[kKaskak]

I just want to point out that that the fix
(process.env.JEST_WORKER_ID != null || process.env.VITEST_WORKER_ID != null) && process.env.NODE_ENV === 'test' is not correct . Its sad that LLM generates this.

This is how expo determines the testing env .
https://github.com/expo/expo/blob/31d4a6cc20eb3c1c6c015ed30b02b9b1b3119769/packages/expo-notifications/src/index.ts#L3-L5
NODE_ENV may or may not be test in JEST

To get it working before removing the package I had to apply your suggestion yes, the other one did not work. Thank you I tagged the wrong person lol