Was wondering if this PR was abandoned. It seems so.

TypeScript may be a better idea nowadays than using Flow.

It appears there's no good email validation library in the ecosystem that works on node, browsers and react native. At least not one that doesn't use regex: https://davidcel.is/posts/stop-validating-email-addresses-with-regex/

The only thing blocking isemail from working in a browser or react native is the util import here:

Additionally, there are some usages of Buffer.byteLength, and react native does not have Buffer so it needs to be polyfilled.

I could come up with a PR that fixes both issues, but I was wondering if it's worth doing it on the current master or if this 4.0 rewrite is still planned.

In the mean time I'm using a quick and dirty patch, and a Buffer polyfill (no need for the cross-context type checking for my purposes):

diff --git a/lib/index.js b/lib/index.js
index 4f3d4cc33d13e68476dbe9ff0021081458af3291..14c2d9a2e83cc0f973f5883030b0e6c34644fe53 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -3,7 +3,6 @@
 // Load modules
 
 const Punycode = require('punycode');
-const Util = require('util');
 
 // Declare internals
 
@@ -219,10 +218,8 @@ if (typeof Symbol !== 'undefined') {
 // Node 10 introduced isSet and isMap, which are useful for cross-context type
 // checking.
 // $lab:coverage:off$
-internals._isSet = (value) => value instanceof Set;
-internals._isMap = (value) => value instanceof Map;
-internals.isSet = Util.types && Util.types.isSet || internals._isSet;
-internals.isMap = Util.types && Util.types.isMap || internals._isMap;
+internals.isSet = (value) => value instanceof Set;
+internals.isMap = (value) => value instanceof Map;
 // $lab:coverage:on$

(related: #157)

I don't have bandwidth for this project (v4 or otherwise), or a real reason to keep working on it. I'd recommend email-addresses if it fits your need. The patch you provided doesn't handle cross-realm objects, so I'm hesitant to say that it's a good way to go for browser usage.