Add "hugo mod npm pack"

This commit also introduces a convention where these common JS config files, including `package.hugo.json`, gets mounted into:

```
assets/_jsconfig
´``

These files mapped to their real filename will be added to the environment when running PostCSS, Babel etc., so you can do `process.env.HUGO_FILE_TAILWIND_CONFIG_JS` to resolve the real filename.

But do note that `assets` is a composite/union filesystem, so if your config file is not meant to be overridden, name them something specific.

This commit also adds adds `workDir/node_modules` to `NODE_PATH` and `HUGO_WORKDIR` to the env when running the JS tools above.

Fixes #7644
Fixes #7656
Fixes #7675

16 files changed, 721 insertions, 46 deletions

diff --git a/commands/mod.go b/commands/mod.go
index 81f660f43..b390d1e75 100644
--- a/commands/mod.go
+++ b/commands/mod.go
@@ -1,4 +1,4 @@
-// Copyright 2019 The Hugo Authors. All rights reserved.
+// Copyright 2020 The Hugo Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -20,6 +20,8 @@ import (
 	"path/filepath"
 	"regexp"
 
+	"github.com/gohugoio/hugo/hugolib"
+
 	"github.com/gohugoio/hugo/modules"
 	"github.com/spf13/cobra"
 )
@@ -114,6 +116,8 @@ This is not needed if you only operate on modules inside /themes or if you have
 		RunE: nil,
 	}
 
+	cmd.AddCommand(newModNPMCmd(c))
+
 	cmd.AddCommand(
 		&cobra.Command{
 			Use:                "get",
@@ -272,6 +276,15 @@ func (c *modCmd) withModsClient(failOnMissingConfig bool, f func(*modules.Client
 	return f(com.hugo().ModulesClient)
 }
 
+func (c *modCmd) withHugo(f func(*hugolib.HugoSites) error) error {
+	com, err := c.initConfig(true)
+	if err != nil {
+		return err
+	}
+
+	return f(com.hugo())
+}
+
 func (c *modCmd) initConfig(failOnNoConfig bool) (*commandeer, error) {
 	com, err := initializeConfig(failOnNoConfig, false, &c.hugoBuilderCommon, c, nil)
 	if err != nil {
diff --git a/commands/mod_npm.go b/commands/mod_npm.go
new file mode 100644
index 000000000..15c875d8e
--- /dev/null
+++ b/commands/mod_npm.go
@@ -0,0 +1,58 @@
+// Copyright 2020 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package commands
+
+import (
+	"github.com/gohugoio/hugo/hugolib"
+	"github.com/gohugoio/hugo/modules/npm"
+	"github.com/spf13/cobra"
+)
+
+func newModNPMCmd(c *modCmd) *cobra.Command {
+
+	cmd := &cobra.Command{
+		Use:   "npm",
+		Short: "Various npm helpers.",
+		Long:  `Various npm (Node package manager) helpers.`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			return c.withHugo(func(h *hugolib.HugoSites) error {
+				return nil
+			})
+		},
+	}
+
+	cmd.AddCommand(&cobra.Command{
+		Use:   "pack",
+		Short: "Experimental: Prepares and writes a composite package.json file for your project.",
+		Long: `Prepares and writes a composite package.json file for your project.
+
+On first run it creates a "package.hugo.json" in the project root if not alread there. This file will be used as a template file
+with the base dependency set. 
+
+This set will be merged with all "package.hugo.json" files found in the dependency tree, picking the version closest to the project.
+
+This command is marked as 'Experimental'. We think it's a great idea, so it's not likely to be
+removed from Hugo, but we need to test this out in "real life" to get a feel of it,
+so this may/will change in future versions of Hugo.
+`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+
+			return c.withHugo(func(h *hugolib.HugoSites) error {
+				return npm.Pack(h.BaseFs.SourceFs, h.BaseFs.Assets.Dirs)
+			})
+		},
+	})
+
+	return cmd
+}
diff --git a/common/hugo/hugo.go b/common/hugo/hugo.go
index 6e07f69c3..ac75e6bca 100644
--- a/common/hugo/hugo.go
+++ b/common/hugo/hugo.go
@@ -17,8 +17,15 @@ import (
 	"fmt"
 	"html/template"
 	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/gohugoio/hugo/hugofs/files"
+
+	"github.com/spf13/afero"
 
 	"github.com/gohugoio/hugo/config"
+	"github.com/gohugoio/hugo/hugofs"
 )
 
 const (
@@ -73,8 +80,23 @@ func NewInfo(environment string) Info {
 	}
 }
 
-func GetExecEnviron(cfg config.Provider) []string {
+func GetExecEnviron(workDir string, cfg config.Provider, fs afero.Fs) []string {
 	env := os.Environ()
+	nodepath := filepath.Join(workDir, "node_modules")
+	if np := os.Getenv("NODE_PATH"); np != "" {
+		nodepath = workDir + string(os.PathListSeparator) + np
+	}
+	config.SetEnvVars(&env, "NODE_PATH", nodepath)
+	config.SetEnvVars(&env, "HUGO_WORKDIR", workDir)
 	config.SetEnvVars(&env, "HUGO_ENVIRONMENT", cfg.GetString("environment"))
+	fis, err := afero.ReadDir(fs, files.FolderJSConfig)
+	if err == nil {
+		for _, fi := range fis {
+			key := fmt.Sprintf("HUGO_FILE_%s", strings.ReplaceAll(strings.ToUpper(fi.Name()), ".", "_"))
+			value := fi.(hugofs.FileMetaInfo).Meta().Filename()
+			config.SetEnvVars(&env, key, value)
+		}
+	}
+
 	return env
 }
diff --git a/docs/content/en/hugo-pipes/babel.md b/docs/content/en/hugo-pipes/babel.md
index 5fb5e11e1..9688626d9 100755
--- a/docs/content/en/hugo-pipes/babel.md
+++ b/docs/content/en/hugo-pipes/babel.md
@@ -24,6 +24,30 @@ Hugo Pipe's Babel requires the `@babel/cli` and `@babel/core` JavaScript package
 If you are using the Hugo Snap package, Babel and plugin(s) need to be installed locally within your Hugo site directory, e.g., `npm install @babel/cli @babel/core --save-dev` without the `-g` flag.
 {{% /note %}}
 
+
+### Config
+
+{{< new-in "v0.75.0" >}}
+
+In Hugo `v0.75` we improved the way we resolve JS configuration and dependencies. One of them is that we now adds the main project's `node_modules` to `NODE_PATH` when running Babel and similar tools. There are some known [issues](https://github.com/babel/babel/issues/5618) with Babel in this area, so if you have a `babel.config.js` living in a Hugo Module (and not in the project itself), we recommend using `require` to load the presets/plugins, e.g.:
+
+
+```js
+module.exports = {
+        presets: [
+                [
+                        require('@babel/preset-env'),
+                        {
+                                useBuiltIns: 'entry',
+                                corejs: 3
+                        }
+                ]
+        ]
+};
+```
+
+
+
 ### Options
 
 config [string]
diff --git a/hugofs/files/classifier.go b/hugofs/files/classifier.go
index 5e26bbac0..35e416c8f 100644
--- a/hugofs/files/classifier.go
+++ b/hugofs/files/classifier.go
@@ -26,6 +26,13 @@ import (
 	"github.com/spf13/afero"
 )
 
+const (
+	// The NPM package.json "template" file.
+	FilenamePackageHugoJSON = "package.hugo.json"
+	// The NPM package file.
+	FilenamePackageJSON = "package.json"
+)
+
 var (
 	// This should be the only list of valid extensions for content files.
 	contentFileExtensions = []string{
@@ -163,9 +170,12 @@ const (
 	ComponentFolderI18n       = "i18n"
 
 	FolderResources = "resources"
+	FolderJSConfig  = "_jsconfig" // Mounted below /assets with postcss.config.js etc.
 )
 
 var (
+	JsConfigFolderMountPrefix = filepath.Join(ComponentFolderAssets, FolderJSConfig)
+
 	ComponentFolders = []string{
 		ComponentFolderArchetypes,
 		ComponentFolderStatic,
diff --git a/hugofs/rootmapping_fs.go b/hugofs/rootmapping_fs.go
index ea3ef003e..2c4f0df52 100644
--- a/hugofs/rootmapping_fs.go
+++ b/hugofs/rootmapping_fs.go
@@ -42,9 +42,6 @@ func NewRootMappingFs(fs afero.Fs, rms ...RootMapping) (*RootMappingFs, error) {
 		(&rm).clean()
 
 		fromBase := files.ResolveComponentFolder(rm.From)
-		if fromBase == "" {
-			panic("unrecognised component folder in" + rm.From)
-		}
 
 		if len(rm.To) < 2 {
 			panic(fmt.Sprintf("invalid root mapping; from/to: %s/%s", rm.From, rm.To))
diff --git a/hugofs/walk_test.go b/hugofs/walk_test.go
index 0c08968c6..c38c6044f 100644
--- a/hugofs/walk_test.go
+++ b/hugofs/walk_test.go
@@ -21,8 +21,6 @@ import (
 	"strings"
 	"testing"
 
-	"github.com/gohugoio/hugo/common/hugo"
-
 	"github.com/pkg/errors"
 
 	"github.com/gohugoio/hugo/htesting"
@@ -129,12 +127,6 @@ func TestWalkSymbolicLink(t *testing.T) {
 	})
 
 	t.Run("BasePath Fs", func(t *testing.T) {
-		if hugo.GoMinorVersion() < 12 {
-			// https://github.com/golang/go/issues/30520
-			// This is fixed in Go 1.13 and in the latest Go 1.12
-			t.Skip("skip this for Go <= 1.11 due to a bug in Go's stdlib")
-
-		}
 		c := qt.New(t)
 
 		docsFs := afero.NewBasePathFs(fs, docsDir)
diff --git a/hugolib/filesystems/basefs.go b/hugolib/filesystems/basefs.go
index 57a95a037..76d49055c 100644
--- a/hugolib/filesystems/basefs.go
+++ b/hugolib/filesystems/basefs.go
@@ -49,6 +49,9 @@ type BaseFs struct {
 	// SourceFilesystems contains the different source file systems.
 	*SourceFilesystems
 
+	// The project source.
+	SourceFs afero.Fs
+
 	// The filesystem used to publish the rendered site.
 	// This usually maps to /my-project/public.
 	PublishFs afero.Fs
@@ -100,6 +103,23 @@ func (b *BaseFs) RelContentDir(filename string) string {
 	return filename
 }
 
+// ResolveJSConfigFile resolves the JS-related config file to a absolute
+// filename. One example of such would be postcss.config.js.
+func (fs *BaseFs) ResolveJSConfigFile(name string) string {
+	// First look in assets/_jsconfig
+	fi, err := fs.Assets.Fs.Stat(filepath.Join(files.FolderJSConfig, name))
+	if err == nil {
+		return fi.(hugofs.FileMetaInfo).Meta().Filename()
+	}
+	// Fall back to the work dir.
+	fi, err = fs.Work.Stat(name)
+	if err == nil {
+		return fi.(hugofs.FileMetaInfo).Meta().Filename()
+	}
+
+	return ""
+}
+
 // SourceFilesystems contains the different source file systems. These can be
 // composite file systems (theme and project etc.), and they have all root
 // set to the source type the provides: data, i18n, static, layouts.
@@ -346,8 +366,10 @@ func NewBase(p *paths.Paths, logger *loggers.Logger, options ...func(*BaseFs) er
 	}
 
 	publishFs := hugofs.NewBaseFileDecorator(afero.NewBasePathFs(fs.Destination, p.AbsPublishDir))
+	sourceFs := hugofs.NewBaseFileDecorator(afero.NewBasePathFs(fs.Source, p.WorkingDir))
 
 	b := &BaseFs{
+		SourceFs:  sourceFs,
 		PublishFs: publishFs,
 	}
 
@@ -696,11 +718,16 @@ type filesystemsCollector struct {
 
 func (c *filesystemsCollector) addDirs(rfs *hugofs.RootMappingFs) {
 	for _, componentFolder := range files.ComponentFolders {
-		dirs, err := rfs.Dirs(componentFolder)
+		c.addDir(rfs, componentFolder)
+	}
 
-		if err == nil {
-			c.overlayDirs[componentFolder] = append(c.overlayDirs[componentFolder], dirs...)
-		}
+}
+
+func (c *filesystemsCollector) addDir(rfs *hugofs.RootMappingFs, componentFolder string) {
+	dirs, err := rfs.Dirs(componentFolder)
+
+	if err == nil {
+		c.overlayDirs[componentFolder] = append(c.overlayDirs[componentFolder], dirs...)
 	}
 }
 
diff --git a/hugolib/hugo_modules_test.go b/hugolib/hugo_modules_test.go
index 0ed4fceb0..c3358a0c2 100644
--- a/hugolib/hugo_modules_test.go
+++ b/hugolib/hugo_modules_test.go
@@ -22,6 +22,8 @@ import (
 	"testing"
 	"time"
 
+	"github.com/gohugoio/hugo/modules/npm"
+
 	"github.com/gohugoio/hugo/common/loggers"
 
 	"github.com/spf13/afero"
@@ -38,7 +40,6 @@ import (
 	"github.com/spf13/viper"
 )
 
-// https://github.com/gohugoio/hugo/issues/6730
 func TestHugoModulesVariants(t *testing.T) {
 	if !isCI() {
 		t.Skip("skip (relative) long running modules test when running locally")
@@ -60,8 +61,10 @@ path="github.com/gohugoio/hugoTestModule2"
 
 	newTestBuilder := func(t testing.TB, moduleOpts string) (*sitesBuilder, func()) {
 		b := newTestSitesBuilder(t)
-		workingDir, clean, err := htesting.CreateTempDir(hugofs.Os, "hugo-modules-variants")
+		tempDir, clean, err := htesting.CreateTempDir(hugofs.Os, "hugo-modules-variants")
 		b.Assert(err, qt.IsNil)
+		workingDir := filepath.Join(tempDir, "myhugosite")
+		b.Assert(os.MkdirAll(workingDir, 0777), qt.IsNil)
 		b.Fs = hugofs.NewDefault(viper.New())
 		b.WithWorkingDir(workingDir).WithConfigFile("toml", createConfig(workingDir, moduleOpts))
 		b.WithTemplates(
@@ -129,6 +132,158 @@ JS imported in module: |
 `)
 	})
 
+	t.Run("Create package.json", func(t *testing.T) {
+
+		b, clean := newTestBuilder(t, "")
+		defer clean()
+
+		b.WithSourceFile("package.json", `{
+		"name": "mypack",
+		"version": "1.2.3",
+        "scripts": {},
+          "dependencies": {
+        	"nonon": "error"
+        	}
+}`)
+
+		b.WithSourceFile("package.hugo.json", `{
+		"name": "mypack",
+		"version": "1.2.3",
+        "scripts": {},
+          "dependencies": {
+        	"foo": "1.2.3"
+        	},
+        "devDependencies": {
+                "postcss-cli": "7.8.0",
+                "tailwindcss": "1.8.0"
+ 
+        }
+}`)
+
+		b.Build(BuildCfg{})
+		b.Assert(npm.Pack(b.H.BaseFs.SourceFs, b.H.BaseFs.Assets.Dirs), qt.IsNil)
+
+		b.AssertFileContentFn("package.json", func(s string) bool {
+			return s == `{
+ "comments": {
+  "dependencies": {
+   "foo": "project",
+   "react-dom": "github.com/gohugoio/hugoTestModule2"
+  },
+  "devDependencies": {
+   "@babel/cli": "github.com/gohugoio/hugoTestModule2",
+   "@babel/core": "github.com/gohugoio/hugoTestModule2",
+   "@babel/preset-env": "github.com/gohugoio/hugoTestModule2",
+   "postcss-cli": "project",
+   "tailwindcss": "project"
+  }
+ },
+ "dependencies": {
+  "foo": "1.2.3",
+  "react-dom": "^16.13.1"
+ },
+ "devDependencies": {
+  "@babel/cli": "7.8.4",
+  "@babel/core": "7.9.0",
+  "@babel/preset-env": "7.9.5",
+  "postcss-cli": "7.8.0",
+  "tailwindcss": "1.8.0"
+ },
+ "name": "mypack",
+ "scripts": {},
+ "version": "1.2.3"
+}`
+		})
+	})
+
+	t.Run("Create package.json, no default", func(t *testing.T) {
+
+		b, clean := newTestBuilder(t, "")
+		defer clean()
+
+		b.WithSourceFile("package.json", `{
+		"name": "mypack",
+		"version": "1.2.3",
+        "scripts": {},
+          "dependencies": {
+           "moo": "1.2.3"
+        	}
+}`)
+
+		b.Build(BuildCfg{})
+		b.Assert(npm.Pack(b.H.BaseFs.SourceFs, b.H.BaseFs.Assets.Dirs), qt.IsNil)
+
+		b.AssertFileContentFn("package.json", func(s string) bool {
+			return s == `{
+ "comments": {
+  "dependencies": {
+   "moo": "project",
+   "react-dom": "github.com/gohugoio/hugoTestModule2"
+  },
+  "devDependencies": {
+   "@babel/cli": "github.com/gohugoio/hugoTestModule2",
+   "@babel/core": "github.com/gohugoio/hugoTestModule2",
+   "@babel/preset-env": "github.com/gohugoio/hugoTestModule2",
+   "postcss-cli": "github.com/gohugoio/hugoTestModule2",
+   "tailwindcss": "github.com/gohugoio/hugoTestModule2"
+  }
+ },
+ "dependencies": {
+  "moo": "1.2.3",
+  "react-dom": "^16.13.1"
+ },
+ "devDependencies": {
+  "@babel/cli": "7.8.4",
+  "@babel/core": "7.9.0",
+  "@babel/preset-env": "7.9.5",
+  "postcss-cli": "7.1.0",
+  "tailwindcss": "1.2.0"
+ },
+ "name": "mypack",
+ "scripts": {},
+ "version": "1.2.3"
+}`
+		})
+	})
+
+	t.Run("Create package.json, no default, no package.json", func(t *testing.T) {
+
+		b, clean := newTestBuilder(t, "")
+		defer clean()
+
+		b.Build(BuildCfg{})
+		b.Assert(npm.Pack(b.H.BaseFs.SourceFs, b.H.BaseFs.Assets.Dirs), qt.IsNil)
+
+		b.AssertFileContentFn("package.json", func(s string) bool {
+			return s == `{
+ "comments": {
+  "dependencies": {
+   "react-dom": "github.com/gohugoio/hugoTestModule2"
+  },
+  "devDependencies": {
+   "@babel/cli": "github.com/gohugoio/hugoTestModule2",
+   "@babel/core": "github.com/gohugoio/hugoTestModule2",
+   "@babel/preset-env": "github.com/gohugoio/hugoTestModule2",
+   "postcss-cli": "github.com/gohugoio/hugoTestModule2",
+   "tailwindcss": "github.com/gohugoio/hugoTestModule2"
+  }
+ },
+ "dependencies": {
+  "react-dom": "^16.13.1"
+ },
+ "devDependencies": {
+  "@babel/cli": "7.8.4",
+  "@babel/core": "7.9.0",
+  "@babel/preset-env": "7.9.5",
+  "postcss-cli": "7.1.0",
+  "tailwindcss": "1.2.0"
+ },
+ "name": "myhugosite",
+ "version": "0.1.0"
+}`
+		})
+	})
+
 }
 
 // TODO(bep) this fails when testmodBuilder is also building ...
diff --git a/hugolib/resource_chain_test.go b/hugolib/resource_chain_test.go
index 64484f1f4..7573199aa 100644
--- a/hugolib/resource_chain_test.go
+++ b/hugolib/resource_chain_test.go
@@ -873,6 +873,10 @@ func TestResourceChainPostCSS(t *testing.T) {
 
 	postcssConfig := `
 console.error("Hugo Environment:", process.env.HUGO_ENVIRONMENT );
+// https://github.com/gohugoio/hugo/issues/7656
+console.error("package.json:", process.env.HUGO_FILE_PACKAGE_JSON );
+console.error("PostCSS Config File:", process.env.HUGO_FILE_POSTCSS_CONFIG_JS );
+
 
 module.exports = {
   plugins: [
@@ -954,6 +958,8 @@ class-in-b {
 
 	// Make sure Node sees this.
 	b.Assert(logBuf.String(), qt.Contains, "Hugo Environment: production")
+	b.Assert(logBuf.String(), qt.Contains, fmt.Sprintf("PostCSS Config File: %s/postcss.config.js", workDir))
+	b.Assert(logBuf.String(), qt.Contains, fmt.Sprintf("package.json: %s/package.json", workDir))
 
 	b.AssertFileContent("public/index.html", `
 Styles RelPermalink: /css/styles.css
diff --git a/modules/collect.go b/modules/collect.go
index b82d395fd..8959572d6 100644
--- a/modules/collect.go
+++ b/modules/collect.go
@@ -18,6 +18,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"regexp"
 	"strings"
 	"time"
 
@@ -382,6 +383,11 @@ func (c *collector) applyMounts(moduleImport Import, mod *moduleAdapter) error {
 		return err
 	}
 
+	mounts, err = c.mountCommonJSConfig(mod, mounts)
+	if err != nil {
+		return err
+	}
+
 	mod.mounts = mounts
 	return nil
 }
@@ -549,6 +555,43 @@ func (c *collector) loadModules() error {
 	return nil
 }
 
+// Matches postcss.config.js etc.
+var commonJSConfigs = regexp.MustCompile(`(babel|postcss|tailwind)\.config\.js`)
+
+func (c *collector) mountCommonJSConfig(owner *moduleAdapter, mounts []Mount) ([]Mount, error) {
+	for _, m := range mounts {
+		if strings.HasPrefix(m.Target, files.JsConfigFolderMountPrefix) {
+			// This follows the convention of the other component types (assets, content, etc.),
+			// if one or more is specificed by the user, we skip the defaults.
+			// These mounts were added to Hugo in 0.75.
+			return mounts, nil
+		}
+	}
+
+	// Mount the common JS config files.
+	fis, err := afero.ReadDir(c.fs, owner.Dir())
+	if err != nil {
+		return mounts, err
+	}
+
+	for _, fi := range fis {
+		n := fi.Name()
+
+		should := n == files.FilenamePackageHugoJSON || n == files.FilenamePackageJSON
+		should = should || commonJSConfigs.MatchString(n)
+
+		if should {
+			mounts = append(mounts, Mount{
+				Source: n,
+				Target: filepath.Join(files.ComponentFolderAssets, files.FolderJSConfig, n),
+			})
+		}
+
+	}
+
+	return mounts, nil
+}
+
 func (c *collector) normalizeMounts(owner *moduleAdapter, mounts []Mount) ([]Mount, error) {
 	var out []Mount
 	dir := owner.Dir()
diff --git a/modules/config.go b/modules/config.go
index 1964479f4..e0a0ea060 100644
--- a/modules/config.go
+++ b/modules/config.go
@@ -56,7 +56,9 @@ func ApplyProjectConfigDefaults(cfg config.Provider, mod Module) error {
 	// the basic level.
 	componentsConfigured := make(map[string]bool)
 	for _, mnt := range moda.mounts {
-		componentsConfigured[mnt.Component()] = true
+		if !strings.HasPrefix(mnt.Target, files.JsConfigFolderMountPrefix) {
+			componentsConfigured[mnt.Component()] = true
+		}
 	}
 
 	type dirKeyComponent struct {
@@ -318,12 +320,21 @@ type Mount struct {
 	Target string // relative target path, e.g. "assets/bootstrap/scss"
 
 	Lang string // any language code associated with this mount.
+
 }
 
 func (m Mount) Component() string {
 	return strings.Split(m.Target, fileSeparator)[0]
 }
 
+func (m Mount) ComponentAndName() (string, string) {
+	k := strings.Index(m.Target, fileSeparator)
+	if k == -1 {
+		return m.Target, ""
+	}
+	return m.Target[:k], m.Target[k+1:]
+}
+
 func getStaticDirs(cfg config.Provider) []string {
 	var staticDirs []string
 	for i := -1; i <= 10; i++ {
diff --git a/modules/npm/package_builder.go b/modules/npm/package_builder.go
new file mode 100644
index 000000000..23aac7246
--- /dev/null
+++ b/modules/npm/package_builder.go
@@ -0,0 +1,230 @@
+// Copyright 2020 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package npm
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+
+	"github.com/gohugoio/hugo/hugofs/files"
+
+	"github.com/pkg/errors"
+
+	"github.com/gohugoio/hugo/hugofs"
+	"github.com/spf13/afero"
+
+	"github.com/spf13/cast"
+
+	"github.com/gohugoio/hugo/helpers"
+)
+
+const (
+	dependenciesKey    = "dependencies"
+	devDependenciesKey = "devDependencies"
+
+	packageJSONName = "package.json"
+
+	packageJSONTemplate = `{
+  "name": "%s",
+  "version": "%s"
+}`
+)
+
+func Pack(fs afero.Fs, fis []hugofs.FileMetaInfo) error {
+
+	var b *packageBuilder
+
+	// Have a package.hugo.json?
+	fi, err := fs.Stat(files.FilenamePackageHugoJSON)
+	if err != nil {
+		// Have a package.json?
+		fi, err = fs.Stat(packageJSONName)
+		if err != nil {
+			// Create one.
+			name := "project"
+			// Use the Hugo site's folder name as the default name.
+			// The owner can change it later.
+			rfi, err := fs.Stat("")
+			if err == nil {
+				name = rfi.Name()
+			}
+			packageJSONContent := fmt.Sprintf(packageJSONTemplate, name, "0.1.0")
+			if err = afero.WriteFile(fs, files.FilenamePackageHugoJSON, []byte(packageJSONContent), 0666); err != nil {
+				return err
+			}
+			fi, err = fs.Stat(files.FilenamePackageHugoJSON)
+			if err != nil {
+				return err
+			}
+		}
+	}
+
+	meta := fi.(hugofs.FileMetaInfo).Meta()
+	masterFilename := meta.Filename()
+	f, err := meta.Open()
+	if err != nil {
+		return errors.Wrap(err, "npm pack: failed to open package file")
+	}
+	b = newPackageBuilder(meta.Module(), f)
+	f.Close()
+
+	for _, fi := range fis {
+		if fi.IsDir() {
+			// We only care about the files in the root.
+			continue
+		}
+
+		if fi.Name() != files.FilenamePackageHugoJSON {
+			continue
+		}
+
+		meta := fi.(hugofs.FileMetaInfo).Meta()
+
+		if meta.Filename() == masterFilename {
+			continue
+		}
+
+		f, err := meta.Open()
+		if err != nil {
+			return errors.Wrap(err, "npm pack: failed to open package file")
+		}
+		b.Add(meta.Module(), f)
+		f.Close()
+	}
+
+	if b.Err() != nil {
+		return errors.Wrap(b.Err(), "npm pack: failed to build")
+	}
+
+	// Replace the dependencies in the original template with the merged set.
+	b.originalPackageJSON[dependenciesKey] = b.dependencies
+	b.originalPackageJSON[devDependenciesKey] = b.devDependencies
+	var commentsm map[string]interface{}
+	comments, found := b.originalPackageJSON["comments"]
+	if found {
+		commentsm = cast.ToStringMap(comments)
+	} else {
+		commentsm = make(map[string]interface{})
+	}
+	commentsm[dependenciesKey] = b.dependenciesComments
+	commentsm[devDependenciesKey] = b.devDependenciesComments
+	b.originalPackageJSON["comments"] = commentsm
+
+	// Write it out to the project package.json
+	packageJSONData, err := json.MarshalIndent(b.originalPackageJSON, "", " ")
+	if err != nil {
+		return errors.Wrap(err, "npm pack: failed to marshal JSON")
+	}
+
+	if err := afero.WriteFile(fs, packageJSONName, packageJSONData, 0666); err != nil {
+		return errors.Wrap(err, "npm pack: failed to write package.json")
+	}
+
+	return nil
+
+}
+
+func newPackageBuilder(source string, first io.Reader) *packageBuilder {
+	b := &packageBuilder{
+		devDependencies:         make(map[string]interface{}),
+		devDependenciesComments: make(map[string]interface{}),
+		dependencies:            make(map[string]interface{}),
+		dependenciesComments:    make(map[string]interface{}),
+	}
+
+	m := b.unmarshal(first)
+	if b.err != nil {
+		return b
+	}
+
+	b.addm(source, m)
+	b.originalPackageJSON = m
+
+	return b
+}
+
+type packageBuilder struct {
+	err error
+
+	// The original package.hugo.json.
+	originalPackageJSON map[string]interface{}
+
+	devDependencies         map[string]interface{}
+	devDependenciesComments map[string]interface{}
+	dependencies            map[string]interface{}
+	dependenciesComments    map[string]interface{}
+}
+
+func (b *packageBuilder) Add(source string, r io.Reader) *packageBuilder {
+	if b.err != nil {
+		return b
+	}
+
+	m := b.unmarshal(r)
+	if b.err != nil {
+		return b
+	}
+
+	b.addm(source, m)
+
+	return b
+}
+
+func (b *packageBuilder) addm(source string, m map[string]interface{}) {
+	if source == "" {
+		source = "project"
+	}
+
+	// The version selection is currently very simple.
+	// We may consider minimal version selection or something
+	// after testing this out.
+	//
+	// But for now, the first version string for a given dependency wins.
+	// These packages will be added by order of import (project, module1, module2...),
+	// so that should at least give the project control over the situation.
+	if devDeps, found := m[devDependenciesKey]; found {
+		mm := cast.ToStringMapString(devDeps)
+		for k, v := range mm {
+			if _, added := b.devDependencies[k]; !added {
+				b.devDependencies[k] = v
+				b.devDependenciesComments[k] = source
+			}
+		}
+	}
+
+	if deps, found := m[dependenciesKey]; found {
+		mm := cast.ToStringMapString(deps)
+		for k, v := range mm {
+			if _, added := b.dependencies[k]; !added {
+				b.dependencies[k] = v
+				b.dependenciesComments[k] = source
+			}
+		}
+	}
+
+}
+
+func (b *packageBuilder) unmarshal(r io.Reader) map[string]interface{} {
+	m := make(map[string]interface{})
+	err := json.Unmarshal(helpers.ReaderToBytes(r), &m)
+	if err != nil {
+		b.err = err
+	}
+	return m
+}
+
+func (b *packageBuilder) Err() error {
+	return b.err
+}
diff --git a/modules/npm/package_builder_test.go b/modules/npm/package_builder_test.go
new file mode 100644
index 000000000..510a04776
--- /dev/null
+++ b/modules/npm/package_builder_test.go
@@ -0,0 +1,95 @@
+// Copyright 2020 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package npm
+
+import (
+	"strings"
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+const templ = `{
+        "name": "foo",
+        "version": "0.1.1",
+        "scripts": {},
+         "dependencies": {
+                "react-dom": "1.1.1",
+                "tailwindcss": "1.2.0",
+                "@babel/cli": "7.8.4",
+                "@babel/core": "7.9.0",
+                "@babel/preset-env": "7.9.5"
+        },
+        "devDependencies": {
+                "postcss-cli": "7.1.0",
+                "tailwindcss": "1.2.0",
+                "@babel/cli": "7.8.4",
+                "@babel/core": "7.9.0",
+                "@babel/preset-env": "7.9.5"
+        }
+}`
+
+func TestPackageBuilder(t *testing.T) {
+	c := qt.New(t)
+
+	b := newPackageBuilder("", strings.NewReader(templ))
+	c.Assert(b.Err(), qt.IsNil)
+
+	b.Add("mymod", strings.NewReader(`{
+"dependencies": {
+	 "react-dom": "9.1.1",
+	 "add1": "1.1.1"
+},
+"devDependencies": {
+	 "tailwindcss": "error",
+	 "add2": "2.1.1"
+}	
+}`))
+
+	b.Add("mymod", strings.NewReader(`{
+"dependencies": {
+	 "react-dom": "error",
+	 "add1": "error",
+	 "add3": "3.1.1"
+},
+"devDependencies": {
+	 "tailwindcss": "error",
+	 "add2": "error",
+	 "add4": "4.1.1"
+	 
+}	
+}`))
+
+	c.Assert(b.Err(), qt.IsNil)
+
+	c.Assert(b.dependencies, qt.DeepEquals, map[string]interface{}{
+		"@babel/cli":        "7.8.4",
+		"add1":              "1.1.1",
+		"add3":              "3.1.1",
+		"@babel/core":       "7.9.0",
+		"@babel/preset-env": "7.9.5",
+		"react-dom":         "1.1.1",
+		"tailwindcss":       "1.2.0",
+	})
+
+	c.Assert(b.devDependencies, qt.DeepEquals, map[string]interface{}{
+		"tailwindcss":       "1.2.0",
+		"@babel/cli":        "7.8.4",
+		"@babel/core":       "7.9.0",
+		"add2":              "2.1.1",
+		"add4":              "4.1.1",
+		"@babel/preset-env": "7.9.5",
+		"postcss-cli":       "7.1.0",
+	})
+}
diff --git a/resources/resource_transformers/babel/babel.go b/resources/resource_transformers/babel/babel.go
index c5ab48dd8..4255210c3 100644
--- a/resources/resource_transformers/babel/babel.go
+++ b/resources/resource_transformers/babel/babel.go
@@ -14,6 +14,7 @@
 package babel
 
 import (
+	"bytes"
 	"io"
 	"os/exec"
 	"path/filepath"
@@ -27,7 +28,6 @@ import (
 	"github.com/mitchellh/mapstructure"
 
 	"github.com/gohugoio/hugo/common/herrors"
-	"github.com/gohugoio/hugo/hugofs"
 	"github.com/gohugoio/hugo/resources"
 	"github.com/gohugoio/hugo/resources/resource"
 	"github.com/pkg/errors"
@@ -120,6 +120,9 @@ func (t *babelTransformation) Transform(ctx *resources.ResourceTransformationCtx
 	var configFile string
 	logger := t.rs.Logger
 
+	var errBuf bytes.Buffer
+	infoW := loggers.LoggerToWriterWithPrefix(logger.INFO, "babel")
+
 	if t.options.Config != "" {
 		configFile = t.options.Config
 	} else {
@@ -130,16 +133,10 @@ func (t *babelTransformation) Transform(ctx *resources.ResourceTransformationCtx
 
 	// We need an abolute filename to the config file.
 	if !filepath.IsAbs(configFile) {
-		// We resolve this against the virtual Work filesystem, to allow
-		// this config file to live in one of the themes if needed.
-		fi, err := t.rs.BaseFs.Work.Stat(configFile)
-		if err != nil {
-			if t.options.Config != "" {
-				// Only fail if the user specificed config file is not found.
-				return errors.Wrapf(err, "babel config %q not found:", configFile)
-			}
-		} else {
-			configFile = fi.(hugofs.FileMetaInfo).Meta().Filename()
+		configFile = t.rs.BaseFs.ResolveJSConfigFile(configFile)
+		if configFile == "" && t.options.Config != "" {
+			// Only fail if the user specificed config file is not found.
+			return errors.Errorf("babel config %q not found:", configFile)
 		}
 	}
 
@@ -158,8 +155,8 @@ func (t *babelTransformation) Transform(ctx *resources.ResourceTransformationCtx
 	cmd := exec.Command(binary, cmdArgs...)
 
 	cmd.Stdout = ctx.To
-	cmd.Stderr = loggers.LoggerToWriterWithPrefix(logger.INFO, "babel")
-	cmd.Env = hugo.GetExecEnviron(t.rs.Cfg)
+	cmd.Stderr = io.MultiWriter(infoW, &errBuf)
+	cmd.Env = hugo.GetExecEnviron(t.rs.WorkingDir, t.rs.Cfg, t.rs.BaseFs.Assets.Fs)
 
 	stdin, err := cmd.StdinPipe()
 	if err != nil {
@@ -173,7 +170,7 @@ func (t *babelTransformation) Transform(ctx *resources.ResourceTransformationCtx
 
 	err = cmd.Run()
 	if err != nil {
-		return err
+		return errors.Wrap(err, errBuf.String())
 	}
 
 	return nil
diff --git a/resources/resource_transformers/postcss/postcss.go b/resources/resource_transformers/postcss/postcss.go
index 258185a40..41472fe98 100644
--- a/resources/resource_transformers/postcss/postcss.go
+++ b/resources/resource_transformers/postcss/postcss.go
@@ -170,17 +170,11 @@ func (t *postcssTransformation) Transform(ctx *resources.ResourceTransformationC
 
 	// We need an abolute filename to the config file.
 	if !filepath.IsAbs(configFile) {
-		// We resolve this against the virtual Work filesystem, to allow
-		// this config file to live in one of the themes if needed.
-		fi, err := t.rs.BaseFs.Work.Stat(configFile)
-		if err != nil {
-			if t.options.Config != "" {
-				// Only fail if the user specificed config file is not found.
-				return errors.Wrapf(err, "postcss config %q not found:", configFile)
-			}
-			configFile = ""
-		} else {
-			configFile = fi.(hugofs.FileMetaInfo).Meta().Filename()
+		configFile = t.rs.BaseFs.ResolveJSConfigFile(configFile)
+		if configFile == "" && t.options.Config != "" {
+			// Only fail if the user specificed config file is not found.
+			return errors.Errorf("postcss config %q not found:", configFile)
+
 		}
 	}
 
@@ -202,7 +196,8 @@ func (t *postcssTransformation) Transform(ctx *resources.ResourceTransformationC
 
 	cmd.Stdout = ctx.To
 	cmd.Stderr = io.MultiWriter(infoW, &errBuf)
-	cmd.Env = hugo.GetExecEnviron(t.rs.Cfg)
+
+	cmd.Env = hugo.GetExecEnviron(t.rs.WorkingDir, t.rs.Cfg, t.rs.BaseFs.Assets.Fs)
 
 	stdin, err := cmd.StdinPipe()
 	if err != nil {