Seems to be working

.eslintrc.json

@@ -0,0 +1,130 @@
+{
+    "env": {
+        "es6": true,
+        "node": true,
+        "browser": true
+    },
+    "extends": [ "eslint:recommended", "plugin:react/recommended", "plugin:react-native/all", "plugin:lodash/recommended" ],
+    "parserOptions": {
+        "ecmaFeatures": {
+            "jsx": true
+        },
+        "ecmaVersion": 2018,
+        "sourceType": "module"
+    },
+    "plugins": [
+        "react",
+        "react-native",
+        "react-filenames",
+        "lodash",
+        "dependencies"
+    ],
+    "rules": {
+        "indent": [ "error", 2 ],
+        "linebreak-style": [ "error", "unix" ],
+        "semi": [ "warn", "always" ],
+        "no-await-in-loop": [ "error" ],
+        "no-prototype-builtins": [ "error" ],
+        "no-template-curly-in-string": [ "warn" ],
+        "array-callback-return": [ "error" ],
+        "block-scoped-var": [ "warn" ],
+        "complexity": [ "warn" ],
+        "consistent-return": [ "error" ],
+        "curly": [ "error", "multi-line" ],
+        "default-case": [ "error" ],
+        "dot-location": [ "warn", "object" ],
+        "dot-notation": [ "warn", {"allowKeywords": true} ],
+        "eqeqeq": [ "error" ],
+        "no-alert": [ "error" ],
+        "no-caller": [ "error" ],
+        "no-div-regex": [ "error" ],
+        "no-eq-null": [ "error" ],
+        "no-eval": [ "error" ],
+        "no-extra-bind": [ "warn" ],
+        "no-extra-label": [ "warn" ],
+        "no-floating-decimal": [ "error" ],
+        "no-implicit-globals": [ "error" ],
+        "no-implied-eval": [ "error" ],
+        "no-invalid-this": [ "error" ],
+        "no-iterator": [ "error" ],
+        "no-lone-blocks": [ "warn" ],
+        "no-loop-func": [ "error" ],
+        "no-magic-numbers": [ "error", { "ignore": [0,1] } ],
+        "no-new-func": [ "error" ],
+        "no-new-wrappers": [ "error" ],
+        "no-proto": [ "error" ],
+        "no-return-assign": [ "error" ],
+        "no-return-await": [ "error" ],
+        "no-self-compare": [ "error" ],
+        "no-sequences": [ "error" ],
+        "no-unmodified-loop-condition": [ "warn" ],
+        "no-unused-expressions": [ "error", { "allowTernary": true, "allowShortCircuit": true } ],
+        "no-useless-call": [ "error" ],
+        "no-useless-return": [ "error" ],
+        "no-void": [ "error" ],
+        "no-with": [ "error" ],
+        "radix": [ "error", "always" ],
+        "require-await": [ "error" ],
+        "strict": [ "error", "global" ],
+        "no-label-var": [ "error" ],
+        "no-shadow": [ "error" ],
+        "no-shadow-restricted-names": [ "error" ],
+        "no-undef-init": [ "error" ],
+        "no-use-before-define": [ "error" ],
+        "no-bitwise": [ "error" ],
+        "no-var": [ "error" ],
+        "prefer-arrow-callback": [ "warn" ],
+        "prefer-const": [ "error" ],
+        "prefer-destructuring": [
+            "error",
+            {"array": false},
+            {"enforceForRenamedProperties": false}
+        ],
+        "prefer-rest-params": [ "warn" ],
+        "prefer-spread": [ "warn" ],
+        "prefer-template": [ "warn" ],
+        "yield-star-spacing": [ "error", "after" ],
+        "callback-return": [ "error" ],
+        "handle-callback-err": [ "error" ],
+        "no-buffer-constructor": [ "error" ],
+        "no-sync": [ "error", {"allowAtRootLevel": true} ],
+        "array-bracket-newline": [ "warn", "consistent" ],
+        "camelcase": [ "warn", { "ignoreDestructuring": true } ],
+        "comma-dangle": [
+            "error",
+            { "arrays": "always-multiline",
+              "objects": "always-multiline",
+              "imports": "always-multiline",
+              "exports": "always-multiline",
+              "functions": "never"
+            }
+        ],
+        "function-paren-newline": [ "warn" ],
+        "implicit-arrow-linebreak": [ "error", "beside" ],
+        "no-mixed-operators": [ "error" ],
+        "no-trailing-spaces": [ "warn", { "ignoreComments": true } ],
+        "no-unneeded-ternary": [ "warn" ],
+        "prefer-object-spread": [ "error" ],
+        "arrow-body-style": [ "error", "as-needed" ],
+        "arrow-parens": [ "error" ],
+        "generator-star-spacing": [ "error", "after" ],
+        "no-confusing-arrow": [ "error" ],
+        "no-duplicate-imports": [ "error", { "includeExports": true } ],
+        "no-useless-computed-key": [ "warn" ],
+        "no-useless-rename": [ "error" ],
+        "no-await-in-loop": [ "error" ],
+        "no-console": [ "error", { "allow": ["debug","warn","error"] } ],
+        "lodash/import-scope": [ "off" ],
+        "lodash/prefer-lodash-method": [ "off" ],
+        "lodash/prop-shorthand": [ "warn" ],
+        "lodash/path-style": [ "warn" ],
+        "template-tag-spacing": [ "warn" ],
+        "react-native/no-inline-styles": [ "warn" ],
+        "react/prop-types": [ "warn" ],
+        "dependencies/case-sensitive": [ "error" ],
+        "dependencies/no-cycles": [ "warn" ],
+        "dependencies/no-unresolved": [ "off" ],
+        "dependencies/require-json-ext": [ "warn" ],
+        "react-filenames/filename-matches-component": [ "off" ]
+    }
+}

README.md

@@ -1,2 +1,39 @@
 # react-native-email-deep-validator
 Goes beyond validating the characters that make up the email, and instead performs checks on the network to ensure that the e-mail address is valid.
+
+# Synposis
+
+```bash
+yarn add react-native-email-validator
+```
+
+```javascript
+import validateEmail from "react-native-email-validator"
+
+await validateEmail("developer+rnev@beewell.health"); // returns true
+await validateEmail("@beewell.health"); // returns false
+await validateEmail("developer+rnev@health"); // returns false
+```
+
+# Validation Phases
+
+## Step 1: Syntax
+
+* It first ensures that the value is a non-empty string, that the character '`@`' is somewhere in that string, and that it is not the first or last character.
+* Then uses [validator/lib/isEmail](https://www.npmjs.com/package/validator) for validating the e-mail address format, which amounts to firing a big ugly regexp against it.
+
+## Step 2: Network Checks
+
+* Calls out to [the `1.1.1.1` DNS over HTTPS server](https://developers.cloudflare.com/1.1.1.1/dns-over-https/) to ensure that there exists at least one MX record for the domain.
+* At the same time, calls out to the [Kickbox "disposable email" API](https://open.kickbox.com/v1/disposable/beewell.health) to validate that the e-mail domain is not a disposable email domain.
+* It'd be nice to do an SMTP check, but it's unclear how to do that from within React Native without unlinking.
+
+Note that any network check that errors out will be counted as a pass. So if there is no internet, the internet connection is very slow, or the server is down, then it is equivalent to the network check returning valid.
+
+# License
+
+MIT. See the file named `LICENSE` in this directory for details.
+
+# Inspiration
+
+[email-deep-validator](https://github.com/getconversio/email-deep-validator/)

index.js

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+
+
+const { isNil, isEmpty, isString, merge, not } = require("lodash");
+const looksLikeEmail = require("validator/lib/isEmail");
+const Promise = require("bluebird");
+const URI = require("urijs");
+
+const atSymbol = "@";
+const getDnsOverHttpUri = (
+  (baseUri) => (domainName) => baseUri.clone().setQuery({name: domainName})
+)(new URI("https://cloudflare-dns.com/dns-query").setQuery({
+  'type': 'MX',
+  'do': true,
+  'cd': false,
+}));
+const getBurnerCheckUri = (
+  (baseUri) => (domainName) => baseUri.clone().filename(domainName)
+)(new URI("https://open.kickbox.com/v1/disposable/beewell.health"));
+
+const doFetch = (uri, customOptions={}) => Promise.try(() => fetch(uri, merge({
+  credentials: 'omit',
+  mode: 'no-cors',
+  method: 'GET',
+}, customOptions)).tap((response) => {
+  if(!response.ok) throw new Error("Network error");
+}).get("body").call("json"));
+
+
+module.exports = (addr) => {
+  const onError = (msg) => (error) => {
+    console.warn(`Error while ${msg}; returning true by default`, { addr, error});
+    return true;
+  };
+  const logResults = (msg) => (result) => console.debug(
+    `Results from ${msg} => ${result}`,
+    { addr, result }
+  );
+
+  const timeLimitMs = 5000;
+
+  const runCheck = (msg, func) => Promise.try(() => func(addr)).timeout(timeLimitMs).catch(onError(msg)).tap(logResults(msg));
+
+  const validatingName = "validating email address";
+  return runCheck(validatingName, () => {
+
+    // Basic sanity checks.
+    if(isNil(addr)) return false;
+    if(!(isString(addr))) return false;
+    if(isEmpty(addr.trim())) return false;
+
+    // Ensure that '@' is not the first or last char.
+    if(addr.startsWith(atSymbol)) return false;
+    if(addr.endsWith(atSymbol)) return false;
+
+    // Now split to get the username and domain name
+    const [username, domainName, ...addrExtra] = addr.split(atSymbol);
+    if(!(isNil(addrExtra) || isEmpty(addrExtra))) return false;
+    if(isNil(username) || isNil(domainName)) return false;
+    if(isEmpty(username.trim()) || isEmpty(domainName.trim())) return false;
+
+    // Construct the checks.
+    const syntaxCheckName = "performing the syntax check";
+    const syntaxCheck = runCheck( syntaxCheckName, looksLikeEmail );
+
+    const mxRecordCheckName = "performing the MX record DNS check";
+    const mxRecordCheck = runCheck(
+      mxRecordCheckName,
+      () => doFetch(
+        getDnsOverHttpUri(domainName),
+        { headers: { accept: 'application/dns-json' } }
+      ).tap((result) => {
+        if(result.Status !== 0) {
+          throw new Error(`Bad status code from DNS query: ${result.Status}`);
+        }
+      }).get("Answer").then((answer) => {
+        if(isNil(answer)) {
+          throw new Error(`No answer returned: ${answer}`);
+        }
+        if(isEmpty(answer)) {
+          return false;
+        }
+        return true;
+      })
+    );
+
+    const burnerName = "performing burner e-mail check";
+    const burnerCheck = runCheck(
+      burnerName,
+      () => doFetch(getBurnerCheckUri(domainName)).get("disposable").then((result) => {
+        if(isNil(result)) {
+          throw new Error(`No result returned: ${result}`);
+        }
+        return !result;
+      })
+    );
+
+    const compilingName = "compiling results";
+    return runCheck(compilingName, () => Promise.filter([syntaxCheck,mxRecordCheck,burnerCheck], not).then((failures) => isNil(failures) || isEmpty(failures)));
+
+  });
+};

package.json

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+{
+  "name": "react-native-email-validator",
+  "version": "1.0.0",
+  "description": "Goes beyond validating the characters that make up the email, and additionally performs DNS and mailbox checks to ensure that the email address is valid.",
+  "main": "index.js",
+  "repository": "git@github.com:BeeWell/react-native-email-deep-validator.git",
+  "author": "Robert Fischer <robert+github@getbeewell.com>",
+  "license": "MIT",
+  "private": false,
+  "sideEffects": false,
+  "dependencies": {
+    "bluebird": "^3.5.1",
+    "lodash": "^4.17.10",
+    "urijs": "^1.19.1",
+    "validator": "^10.4.0"
+  },
+  "devDependencies": {
+    "eslint": "^5.1.0",
+    "eslint-plugin-dependencies": "^2.4.0",
+    "eslint-plugin-lodash": "^2.7.0",
+    "eslint-plugin-react": "^7.10.0",
+    "eslint-plugin-react-filenames": "^0.0.2",
+    "eslint-plugin-react-native": "^3.2.1"
+  },
+  "scripts": {
+    "eslint": "eslint --cache --fix ./index.js 2>&1 1>/dev/null || eslint --cache --color --report-unused-disable-directives ./index.js"
+  }
+}