About 2 years ago, I hacked PouchDB@6 to make it able to run on React Native with attachments support. But the current latest version is 7 now.
Unfortunately, the PouchDB community is not interested in supporting React Native.
So, we have to hack it again for getting v7 to work on RN.

This is the work for building my app called Inkdrop - a cross-platform Markdown note-taking app that uses PouchDB for accomplishing its data sync feature.

How to use

Check out the fork repository:

craftzdog / pouchdb-react-native

🐨 - PouchDB is a pocket-sized database, with some patches for running on React Native

PouchDB for React Native

Name: Created PouchDB@7 for React Native
Rating: 1.3 (3065 reviews)
Author: craftzdog

A performant fork of PouchDB for React Native.

NOTE: The attachment support has been dropped since I no longer store binary data in PouchDB.

How to use PouchDB on React Native with SQLite

Install

Install dev package:

yarn add -D babel-plugin-module-resolver

Install polyfill packages:

yarn add events react-native-get-random-values react-native-quick-base64

Install pouchdb packages:

yarn add pouchdb-core pouchdb-replication pouchdb-mapreduce pouchdb-adapter-http

Install patched package:
```
yarn add @craftzdog/pouchdb-collate-react-native
```
It avoids using '\u0000' chars for indexing (See this commit).
Install storage adapter packages:
```
yarn add pouchdb-adapter-react-native-sqlite react-native-quick-sqlite react-native-quick-websql
```
- react-native-quick-sqlite - A fast bindings via JSI for SQLite
- react-native-quick-websql - WebSQL wrapper for quick-sqlite
- pouchdb-adapter-react-native-sqlite - PouchDB adapter for SQLite with those two modules
Install CocoaPods:
```
npx pod-install
```

Configure

Make a shim.js:

import {shim} from 'react-native-quick-base64'

shim()

// Avoid using node dependent modules
process.browser = true

then, require it at the beginning of…

View on GitHub

Also check out a small example.

Install

Install dev package:

yarn add -D babel-plugin-module-resolver

Install polyfill packages:

yarn add events process base-64 react-native-get-random-values react-native-quick-md5

Install pouchdb packages:

yarn add @craftzdog/pouchdb-core-react-native @craftzdog/pouchdb-binary-utils-react-native pouchdb-adapter-http pouchdb-mapreduce pouchdb-replication react-native-pouchdb-md5

Note: @craftzdog/pouchdb-replication-react-native is no longer needed.

Install storage adapter packages:

yarn add pouchdb-adapter-react-native-sqlite react-native-sqlite-2

Install CocoaPods:

cd ios && pod install

Configure

Make a shim.js:

import "react-native-get-random-values";
import { decode, encode } from "base-64";

if (typeof process === "undefined") {
 global.process = require("process");
} else {
 const bProcess = require("process");
 for (var p in bProcess) {
   if (!(p in process)) {
     process[p] = bProcess[p];
   }
 }
}

if (!global.btoa) {
 global.btoa = encode;
}

if (!global.atob) {
 global.atob = decode;
}

process.browser = true;

then, require it at the beginning of your index.js.

Edit your babel.config.js like so:

module.exports = {
 presets: ['module:metro-react-native-babel-preset'],
 plugins: [
   [
     'module-resolver',
     {
       alias: {
         'pouchdb-md5': 'react-native-pouchdb-md5',
         'pouchdb-binary-utils':
           '@craftzdog/pouchdb-binary-utils-react-native',
       },
     },
   ],
 ],
};

Hacking PouchDB (Again)

You can check out what changes have been added to the fork.

React Native has been evolving these 2 years, but FileReader.readAsArrayBuffer is still not implemented.
Creating blobs from ArrayBuffer or ArrayBufferView is not supported as well. I tried to make RN support them by looking into their source code but found that it's hard by design.
So, to deal with attachments in your databases, you have to avoid calling these APIs in PouchDB.

So, you have to hack pouchdb-binary-utils as following.
blobOrBufferToBinaryString calls FileReader.readAsArrayBuffer. Call FileReader.readAsDataURL instead:

--- a/packages/node_modules/pouchdb-binary-utils/src/blobOrBufferToBase64-browser.js
+++ b/packages/node_modules/pouchdb-binary-utils/src/blobOrBufferToBase64-browser.js
@@ -1,10 +1,16 @@
-import { btoa } from './base64';
-import blobOrBufferToBinaryString from './blobOrBufferToBinaryString';
-
 function blobToBase64(blobOrBuffer, callback) {
-  blobOrBufferToBinaryString(blobOrBuffer, function (base64) {
-    callback(btoa(base64));
-  });
+  if (blobOrBuffer.size) {
+    var reader = new FileReader();
+    reader.onloadend = function (e) {
+      const text = e.target.result || '';
+      const base64 = text.split(',')[1];
+      callback(base64);
+    };
+    reader.readAsDataURL(blobOrBuffer);
+  } else {
+    const base64 = blobOrBuffer.toString('base64');
+    setImmediate(() => callback(base64));
+  }
 }

In readAsBinaryString, use blobToBase64 instead of readAsArrayBuffer:

--- a/packages/node_modules/pouchdb-binary-utils/src/readAsBinaryString.js
+++ b/packages/node_modules/pouchdb-binary-utils/src/readAsBinaryString.js
@@ -1,32 +1,10 @@
-//Can't find original post, but this is close
-//http://stackoverflow.com/questions/6965107/ (continues on next line)
-//converting-between-strings-and-arraybuffers
-function arrayBufferToBinaryString(buffer) {
-  var binary = '';
-  var bytes = new Uint8Array(buffer);
-  var length = bytes.byteLength;
-  for (var i = 0; i < length; i++) {
-    binary += String.fromCharCode(bytes[i]);
-  }
-  return binary;
-}
+import blobToBase64 from './blobOrBufferToBase64';

-// shim for browsers that don't support it
 function readAsBinaryString(blob, callback) {
-  var reader = new FileReader();
-  var hasBinaryString = typeof reader.readAsBinaryString === 'function';
-  reader.onloadend = function (e) {
-    var result = e.target.result || '';
-    if (hasBinaryString) {
-      return callback(result);
-    }
-    callback(arrayBufferToBinaryString(result));
-  };
-  if (hasBinaryString) {
-    reader.readAsBinaryString(blob);
-  } else {
-    reader.readAsArrayBuffer(blob);
-  }
+  blobToBase64(blob, (base64) => {
+    const binaryString = atob(base64);
+    callback(binaryString);
+  });
 }

 export default readAsBinaryString;

PouchDB tries to use Blob but it does not work as expected due to lack of Blob support in RN. So, we have to deal with attachments always in base64:

--- a/packages/node_modules/pouchdb-core/src/adapter.js
+++ b/packages/node_modules/pouchdb-core/src/adapter.js
@@ -720,7 +720,7 @@ AbstractPouchDB.prototype.getAttachment =
     }
     if (res.doc._attachments && res.doc._attachments[attachmentId]) {
       opts.ctx = res.ctx;
-      opts.binary = true;
+      opts.binary = false;
       self._getAttachment(docId, attachmentId,
                           res.doc._attachments[attachmentId], opts, callback);
     } else {

pouchdb-md5 also calls readAsArrayBuffer.
I created react-native-pouchdb-md5 which natively calculates MD5 hashes.

Hope you find it useful and helpful.