Use env variable to disable source maps (facebook#2818)

* use env variable to disable source maps

* Rename to use SOURCE_MAP with explicit value

* Update webpack.config.prod.js

packages/react-scripts/config/webpack.config.prod.js

@@ -41,6 +41,8 @@ const publicPath = paths.servedPath;
 // Some apps do not use client-side routing with pushState.
 // For these, "homepage" can be set to "." to enable relative asset paths.
 const shouldUseRelativeAssetPaths = publicPath === './';
+// Source maps are resource heavy and can cause out of memory issue for large source files.
+const shouldUseSourceMap = process.env.GENERATE_SOURCEMAP !== 'false';
 // `publicUrl` is just like `publicPath`, but we will provide it to our app
 // as %PUBLIC_URL% in `index.html` and `process.env.PUBLIC_URL` in JavaScript.
 // Omit trailing slash as %PUBLIC_URL%/xyz looks better than %PUBLIC_URL%xyz.
@@ -74,7 +76,7 @@ module.exports = {
   bail: true,
   // We generate sourcemaps in production. This is slow but gives good results.
   // You can exclude the *.map files from the build during deployment.
-  devtool: 'source-map',
+  devtool: shouldUseSourceMap ? 'source-map' : false,
   // In production, we only want to load the polyfills and the app code.
   entry: [require.resolve('./polyfills'), paths.appIndexJs],
   output: {
@@ -216,7 +218,7 @@ module.exports = {
                       options: {
                         importLoaders: 1,
                         minimize: true,
-                        sourceMap: true,
+                        sourceMap: shouldUseSourceMap,
                       },
                     },
                     {
@@ -307,7 +309,7 @@ module.exports = {
         // https://github.com/facebookincubator/create-react-app/issues/2488
         ascii_only: true,
       },
-      sourceMap: true,
+      sourceMap: shouldUseSourceMap,
     }),
     // Note: this won't work without ExtractTextPlugin.extract(..) in `loaders`.
     new ExtractTextPlugin({