Generate sourcemaps for production build artifacts

Author: markerikson

scripts/rollup/build.js

@@ -10,6 +10,8 @@ const replace = require('@rollup/plugin-replace');
 const stripBanner = require('rollup-plugin-strip-banner');
 const chalk = require('chalk');
 const resolve = require('@rollup/plugin-node-resolve').nodeResolve;
+const MagicString = require('magic-string');
+const remapping = require('@ampproject/remapping');
 const fs = require('fs');
 const argv = require('minimist')(process.argv.slice(2));
 const Modules = require('./modules');
@@ -148,6 +150,7 @@ function getBabelConfig(
     presets: [],
     plugins: [...babelPlugins],
     babelHelpers: 'bundled',
+    sourcemap: false,
   };
   if (isDevelopment) {
     options.plugins.push(
@@ -381,6 +384,21 @@ function getPlugins(
 
   const {isUMDBundle, shouldStayReadable} = getBundleTypeFlags(bundleType);
 
+  const needsMinifiedByClosure = isProduction && bundleType !== ESM_PROD;
+
+  // Only generate sourcemaps for true "production" build artifacts
+  // that will be used by bundlers, such as `react-dom.production.min.js`.
+  // UMD and "profiling" builds are rarely used and not worth having sourcemaps.
+  const needsSourcemaps =
+    needsMinifiedByClosure &&
+    !isProfiling &&
+    !isUMDBundle &&
+    !shouldStayReadable;
+
+  // For builds with sourcemaps, capture the minified code Closure generated
+  // so it can be used to help construct the final sourcemap contents.
+  let chunkCodeAfterClosureCompiler = undefined;
+
   return [
     // Keep dynamic imports as externals
     dynamicImports(),
@@ -390,7 +408,7 @@ function getPlugins(
         const transformed = flowRemoveTypes(code);
         return {
           code: transformed.toString(),
-          map: transformed.generateMap(),
+          map: null,
         };
       },
     },
@@ -419,6 +437,7 @@ function getPlugins(
     ),
     // Remove 'use strict' from individual source files.
     {
+      name: "remove 'use strict'",
       transform(source) {
         return source.replace(/['"]use strict["']/g, '');
       },
@@ -440,35 +459,44 @@ function getPlugins(
     isUMDBundle && entry === 'react-art' && commonjs(),
     // Apply dead code elimination and/or minification.
     // closure doesn't yet support leaving ESM imports intact
-    isProduction &&
-      bundleType !== ESM_PROD &&
-      closure({
-        compilation_level: 'SIMPLE',
-        language_in: 'ECMASCRIPT_2020',
-        language_out:
-          bundleType === NODE_ES2015
-            ? 'ECMASCRIPT_2020'
-            : bundleType === BROWSER_SCRIPT
-            ? 'ECMASCRIPT5'
-            : 'ECMASCRIPT5_STRICT',
-        emit_use_strict:
-          bundleType !== BROWSER_SCRIPT &&
-          bundleType !== ESM_PROD &&
-          bundleType !== ESM_DEV,
-        env: 'CUSTOM',
-        warning_level: 'QUIET',
-        apply_input_source_maps: false,
-        use_types_for_optimization: false,
-        process_common_js_modules: false,
-        rewrite_polyfills: false,
-        inject_libraries: false,
-        allow_dynamic_import: true,
-
-        // Don't let it create global variables in the browser.
-        // https://github.com/facebook/react/issues/10909
-        assume_function_wrapper: !isUMDBundle,
-        renaming: !shouldStayReadable,
-      }),
+    needsMinifiedByClosure &&
+      closure(
+        {
+          compilation_level: 'SIMPLE',
+          language_in: 'ECMASCRIPT_2020',
+          language_out:
+            bundleType === NODE_ES2015
+              ? 'ECMASCRIPT_2020'
+              : bundleType === BROWSER_SCRIPT
+              ? 'ECMASCRIPT5'
+              : 'ECMASCRIPT5_STRICT',
+          emit_use_strict:
+            bundleType !== BROWSER_SCRIPT &&
+            bundleType !== ESM_PROD &&
+            bundleType !== ESM_DEV,
+          env: 'CUSTOM',
+          warning_level: 'QUIET',
+          source_map_include_content: true,
+          use_types_for_optimization: false,
+          process_common_js_modules: false,
+          rewrite_polyfills: false,
+          inject_libraries: false,
+          allow_dynamic_import: true,
+
+          // Don't let it create global variables in the browser.
+          // https://github.com/facebook/react/issues/10909
+          assume_function_wrapper: !isUMDBundle,
+          renaming: !shouldStayReadable,
+        },
+        {needsSourcemaps}
+      ),
+    needsSourcemaps && {
+      name: 'chunk-after-closure',
+      renderChunk(code, config, options) {
+        // Side effect - grab the code as Closure mangled it
+        chunkCodeAfterClosureCompiler = code;
+      },
+    },
     // Add the whitespace back if necessary.
     shouldStayReadable &&
       prettier({
@@ -479,6 +507,7 @@ function getPlugins(
       }),
     // License and haste headers, top-level `if` blocks.
     {
+      name: 'license-and-headers',
       renderChunk(source) {
         return Wrappers.wrapBundle(
           source,
@@ -490,6 +519,69 @@ function getPlugins(
         );
       },
     },
+    needsSourcemaps && {
+      name: 'generate-prod-bundle-sourcemaps',
+      async renderChunk(codeAfterLicense, chunk, options, meta) {
+        // We want to generate a sourcemap that shows the production bundle source
+        // as it existed before Closure Compiler minified that chunk.
+        // We also need to apply any license/wrapper text adjustments to that
+        // sourcemap, so that the mapped locations line up correctly.
+
+        // We can split the final chunk code to figure out what got added around
+        // the code from the Closure step.
+        const [licensePrefix, licensePostfix] = codeAfterLicense.split(
+          chunkCodeAfterClosureCompiler
+        );
+
+        const transformedSource = new MagicString(
+          chunkCodeAfterClosureCompiler
+        );
+
+        // Apply changes so we can generate a sourcemap for this step
+        if (licensePrefix) {
+          transformedSource.prepend(licensePrefix);
+        }
+
+        if (licensePostfix) {
+          transformedSource.append(licensePostfix);
+        }
+
+        // Use a path like `node_modules/react/cjs/react.production.min.js.map` for the sourcemap file
+        const finalSourcemapPath = options.file.replace('.js', '.js.map');
+
+        // Read the sourcemap that Closure wrote to disk
+        const sourcemapAfterClosure = JSON.parse(
+          fs.readFileSync(finalSourcemapPath, 'utf8')
+        );
+
+        // Use a name like `react.production.js` for the "pre-minified" sourcemap contents
+        const fileWithoutMin = filename.replace('.min', '');
+
+        // CC generated a file list that only contains the tempfile name.
+        // Replace that with a more meaningful "source" name for this bundle.
+        sourcemapAfterClosure.sources = [fileWithoutMin];
+        sourcemapAfterClosure.file = filename;
+
+        // Create an additional sourcemap adjusted for the license header contents
+        const mapAfterLicense = transformedSource.generateMap({
+          file: filename,
+          includeContent: true,
+          hires: true,
+        });
+
+        // Merge the Closure sourcemap and the with-license sourcemap together
+        const finalCombinedSourcemap = remapping(
+          [mapAfterLicense, sourcemapAfterClosure],
+          () => null
+        );
+
+        // Overwrite the Closure-generated file with the final combined sourcemap
+        fs.writeFileSync(
+          finalSourcemapPath,
+          JSON.stringify(finalCombinedSourcemap)
+        );
+      },
+    },
     // Record bundle size.
     sizes({
       getSize: (size, gzip) => {

scripts/rollup/plugins/closure-plugin.js

@@ -19,15 +19,25 @@ function compile(flags) {
   });
 }
 
-module.exports = function closure(flags = {}) {
+module.exports = function closure(flags = {}, {needsSourcemaps}) {
   return {
     name: 'scripts/rollup/plugins/closure-plugin',
-    async renderChunk(code) {
+    async renderChunk(code, chunk, options) {
       const inputFile = tmp.fileSync();
-      const tempPath = inputFile.name;
-      flags = Object.assign({}, flags, {js: tempPath});
-      await writeFileAsync(tempPath, code, 'utf8');
-      const compiledCode = await compile(flags);
+
+      // Use a path like `node_modules/react/cjs/react.production.min.js.map` for the sourcemap file
+      const sourcemapPath = options.file.replace('.js', '.js.map');
+
+      // Tell Closure what JS source file to read, and optionally what sourcemap file to write
+      const finalFlags = {
+        ...flags,
+        js: inputFile.name,
+        ...(needsSourcemaps && {create_source_map: sourcemapPath}),
+      };
+
+      await writeFileAsync(inputFile.name, code, 'utf8');
+      const compiledCode = await compile(finalFlags);
+
       inputFile.removeCallback();
       return {code: compiledCode};
     },