Tweak how NODE_PATH is handled

Author: gaearon

config/paths.js

@@ -11,13 +11,17 @@
 
 var path = require('path');
 
-// We support resolving modules according to NODE_PATH.
+// We support resolving modules according to `NODE_PATH`.
 // This lets you use absolute paths in imports inside large monorepos:
 // https://github.com/facebookincubator/create-react-app/issues/253.
-// It works just like NODE_PATH in Node:
+
+// It works similar to `NODE_PATH` in Node itself:
 // https://nodejs.org/api/modules.html#modules_loading_from_the_global_folders
+
 // We will export `nodePaths` as an array of absolute paths.
-// It will then be used by Webpack (and potentially other tools).
+// It will then be used by Webpack configs.
+// Jest doesn’t need this because it already handles `NODE_PATH` out of the box.
+
 var nodePaths = (process.env.NODE_PATH || '')
   .split(process.platform === 'win32' ? ';' : ':')
   .filter(Boolean)

config/webpack.config.dev.js

@@ -67,11 +67,12 @@ module.exports = {
     publicPath: '/'
   },
   resolve: {
-    // This allows you to set a root for where Webpack should look for modules.
-    // It must be an absolute path or an array of absolute paths.
-    // This lets you use absolute paths in imports inside large monorepos:
-    // https://github.com/facebookincubator/create-react-app/issues/253.
-    root: paths.nodePaths,
+    // This allows you to set a fallback for where Webpack should look for modules.
+    // We read `NODE_PATH` environment variable in `paths.js` and pass paths here.
+    // We use `fallback` instead of `root` because we want `node_modules` to "win"
+    // if there any conflicts. This matches Node resolution mechanism.
+    // https://github.com/facebookincubator/create-react-app/issues/253
+    fallback: paths.nodePaths,
     // These are the reasonable defaults supported by the Node ecosystem.
     extensions: ['.js', '.json', ''],
     alias: {

config/webpack.config.prod.js

@@ -62,11 +62,12 @@ module.exports = {
     publicPath: publicPath
   },
   resolve: {
-    // This allows you to set a root for where Webpack should look for modules.
-    // It must be an absolute path or an array of absolute paths.
-    // This lets you use absolute paths in imports inside large monorepos:
-    // https://github.com/facebookincubator/create-react-app/issues/253.
-    root: paths.nodePaths,
+    // This allows you to set a fallback for where Webpack should look for modules.
+    // We read `NODE_PATH` environment variable in `paths.js` and pass paths here.
+    // We use `fallback` instead of `root` because we want `node_modules` to "win"
+    // if there any conflicts. This matches Node resolution mechanism.
+    // https://github.com/facebookincubator/create-react-app/issues/253
+    fallback: paths.nodePaths,
     // These are the reasonable defaults supported by the Node ecosystem.
     extensions: ['.js', '.json', ''],
     alias: {