fix: lazy load getRandomValues (#537)

Without this change, React Native projects must manage an implicit
dependency on `react-native-get-random-values`, and try to ensure that
it gets loaded before `uuid` does.

With this change, the React Native polyfill simply needs to be installed
sometime before the first uuid is created.

Fixes #536

src/rng-browser.js

@@ -2,23 +2,27 @@
 // require the crypto API and do not support built-in fallback to lower quality random number
 // generators (like Math.random()).
 
-// getRandomValues needs to be invoked in a context where "this" is a Crypto implementation. Also,
-// find the complete implementation of crypto (msCrypto) on IE11.
-const getRandomValues =
-  (typeof crypto !== 'undefined' &&
-    crypto.getRandomValues &&
-    crypto.getRandomValues.bind(crypto)) ||
-  (typeof msCrypto !== 'undefined' &&
-    typeof msCrypto.getRandomValues === 'function' &&
-    msCrypto.getRandomValues.bind(msCrypto));
+let getRandomValues;
 
 const rnds8 = new Uint8Array(16);
 
 export default function rng() {
+  // lazy load so that environments that need to polyfill have a chance to do so
   if (!getRandomValues) {
-    throw new Error(
-      'crypto.getRandomValues() not supported. See https://github.com/uuidjs/uuid#getrandomvalues-not-supported'
-    );
+    // getRandomValues needs to be invoked in a context where "this" is a Crypto implementation. Also,
+    // find the complete implementation of crypto (msCrypto) on IE11.
+    getRandomValues =
+      (typeof crypto !== 'undefined' &&
+        crypto.getRandomValues &&
+        crypto.getRandomValues.bind(crypto)) ||
+      (typeof msCrypto !== 'undefined' &&
+        typeof msCrypto.getRandomValues === 'function' &&
+        msCrypto.getRandomValues.bind(msCrypto));
+    if (!getRandomValues) {
+      throw new Error(
+        'crypto.getRandomValues() not supported. See https://github.com/uuidjs/uuid#getrandomvalues-not-supported'
+      );
+    }
   }
 
   return getRandomValues(rnds8);