url [nfc]: Factor out helper tryParseUrl.

This allows callers like `canUseNativeAppleFlow` to use plain
conditionals instead of try/catch, and use more `const` so that
the type-checker is better able to see when a variable can no longer
be a value like `undefined`.

In this case, for example, if instead of this change a `return false`
is simply added to the `catch` block, then Flow still believes the
`host !== undefined` check below is necessary (and that without it
the `host.endsWith(…)` call is ill-typed), because `host` is a
mutable binding and Flow doesn't see how to prove that it doesn't get
set back to `undefined`.  With `const` everywhere, the logic to rule
out such possibilities is more straightforward and the type-checker
is able to see it.

Author: gnprice

src/start/AuthScreen.js

@@ -28,7 +28,7 @@ import styles from '../styles';
 import { Centerer, Screen, ZulipButton } from '../common';
 import { getCurrentRealm } from '../selectors';
 import RealmInfo from './RealmInfo';
-import { getFullUrl, encodeParamsForUrl } from '../utils/url';
+import { getFullUrl, encodeParamsForUrl, tryParseUrl } from '../utils/url';
 import * as webAuth from './webAuth';
 import { loginSuccess, navigateToDev, navigateToPassword } from '../actions';
 import IosCompliantAppleAuthButton from './IosCompliantAppleAuthButton';
@@ -276,12 +276,11 @@ class AuthScreen extends PureComponent<Props> {
       return false;
     }
 
-    let host: string | void;
-    try {
-      host = new WhatwgURL(this.props.realm).host;
-    } catch (e) {
+    const host = tryParseUrl(this.props.realm)?.host;
+    if (host === undefined) {
       // `this.props.realm` invalid.
       // TODO: Check this much sooner.
+      return false;
     }
 
     // The native flow for Apple auth assumes that the app and the server
@@ -291,9 +290,7 @@ class AuthScreen extends PureComponent<Props> {
     //
     // (For other realms, we'll simply fall back to the web flow, which
     // handles things appropriately without relying on that assumption.)
-    return config.appOwnDomains.some(
-      domain => host !== undefined && (host === domain || host.endsWith(`.${domain}`)),
-    );
+    return config.appOwnDomains.some(domain => host === domain || host.endsWith(`.${domain}`));
   };
 
   handleAuth = async (method: AuthenticationMethodDetails) => {

src/utils/url.js

@@ -1,5 +1,6 @@
 /* @flow strict-local */
 import urlRegex from 'url-regex';
+import { URL as WhatwgURL } from 'react-native-url-polyfill';
 
 import type { Auth } from '../types';
 import { getAuthHeaders } from '../api/transport';
@@ -31,6 +32,15 @@ export const encodeParamsForUrl = (params: UrlParams): string =>
     .map(([key, value]) => `${encodeURIComponent(key)}=${encodeURIComponent(value.toString())}`)
     .join('&');
 
+/** Just like `new WhatwgURL`, but on error return undefined instead of throwing. */
+export const tryParseUrl = (url: string, base?: string | WhatwgURL): WhatwgURL | void => {
+  try {
+    return new WhatwgURL(url, base);
+  } catch (e) {
+    return undefined;
+  }
+};
+
 /**
  * Turn a relative or absolute URL into an absolute URL.
  *