Improve realm constructor logic (#742)

* Add user file

* Clean up initialization and add error message

* Revert "Add user file"

This reverts commit 2948f4cfc2dfd2d5d75594307b1e89806b817eb7.

* Make index.js more robust

* Fix review comments

lib/index.js

@@ -18,43 +18,68 @@
 
 'use strict';
 
-function node_require(module) {
+// Prevent React Native packager from seeing modules required with this
+function nodeRequire(module) {
     return require(module);
 }
 
-// If process is defined, we're running in node.
-function isNode() {
-    return typeof process == 'object' && (('' + process) == '[object process]' || typeof jest == 'object')
+
+function getContext() {
+    if (typeof process === 'object' && process + '' === '[object process]') {
+        return process.type === 'renderer' ? 'electron' : 'nodejs';
     }        
 
-// function isElectronRenderer() {
-//     return isNode() && process.type === 'renderer'
-// }
+    if (typeof navigator !== 'undefined') {
+        if (navigator.product === 'ReactNative') {
+            return 'reactnative';
+        }
+
+        if (/Chrome/.test(navigator.userAgent)) {
+            return 'chromedebugger';
+        }
+    }
+
+    if (typeof global !== 'undefined') {
+        if (global.__debug__) {
+            return 'vscodedebugger';
+        }
+    }
+
+    if (typeof Realm !== 'undefined') {
+        return 'jscore';
+    }
+
+    throw Error("Unknown execution context");
+}
 
 var realmConstructor;
-if (isNode()) {
-    node_require('./submit-analytics')('Run'); 
 
-    // Prevent React Native packager from seeing this module.
-    var binary = node_require('node-pre-gyp');
-    var path = node_require('path');
+switch(getContext()) {
+    case 'nodejs':
+    case 'electron':
+        nodeRequire('./submit-analytics')('Run'); 
+
+        var binary = nodeRequire('node-pre-gyp');
+        var path = nodeRequire('path');
         var pkg = path.resolve(path.join(__dirname,'../package.json'));
         var binding_path = binary.find(pkg);
+
         realmConstructor = require(binding_path).Realm;
-} else {
-    if (typeof Realm != 'undefined') {
-        // The global Realm constructor should be available on device (using JavaScriptCore).
+        break;
+    
+    case 'reactnative':
+    case 'jscore':
         realmConstructor = Realm;  // eslint-disable-line no-undef
-    // eslint-disable-next-line
-    } else if (typeof window != 'undefined') {
-        // The userAgent will be defined when running in a browser (such as Chrome debugging mode).
+        break;
+
+    case 'chromedebugger':
+    case 'vscodedebugger':
         realmConstructor = require('./browser').default; // (exported as ES6 module)
-    // eslint-disable-next-line
-    }
+        break;
 }
 
 if (!realmConstructor) {
-    throw new Error('Missing Realm constructor - please ensure RealmReact framework is included!');
+    throw new Error('Missing Realm constructor. Did you run "react-native link realm"? Please see https://realm.io/docs/react-native/latest/#missing-realm-constructor for troubleshooting');
 }
 
 require('./extensions')(realmConstructor);