In this example we add masonry, imagesloaded, and dropzone libraries to Composer.

Composer JS libraries example.diff

diff --git a/.gitignore b/.gitignore
index fc00ed5..ac82bec 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,6 +5,7 @@ web/core
 web/modules/contrib
 web/themes/contrib
 web/profiles/contrib
+web/libraries
 
 # Ignore configuration files that may contain environment-sensitive information.
 web/sites/*/settings.local.php
diff --git a/composer.json b/composer.json
index 476a87e..fdca230 100644
--- a/composer.json
+++ b/composer.json
@@ -108,6 +108,66 @@
             }
         },
         {
+            "type": "package",
+            "package": {
+                "name": "enyo/dropzone",
+                "version": "4.3",
+                "type": "drupal-library",
+                "source": {
+                    "url": "https://github.com/enyo/dropzone.git",
+                    "type": "git",
+                    "reference": "master"
+                },
+                "dist": {
+                    "url": "https://github.com/enyo/dropzone/archive/v4.3.0.zip",
+                    "type": "zip"
+                },
+                "require": {
+                    "composer/installers": "^1.0.20"
+                }
+            }
+        },
+        {
+            "type": "package",
+            "package": {
+                "name": "desandro/masonry",
+                "version": "4.1",
+                "type": "drupal-library",
+                "source": {
+                    "url": "https://github.com/desandro/masonry.git",
+                    "type": "git",
+                    "reference": "master"
+                },
+                "dist": {
+                    "url": "https://github.com/desandro/masonry/archive/v4.1.0.zip",
+                    "type": "zip"
+                },
+                "require": {
+                    "composer/installers": "^1.0.20"
+                }
+            }
+        },
+        {
+            "type": "package",
+            "package": {
+                "name": "desandro/imagesloaded",
+                "version": "4.1",
+                "type": "drupal-library",
+                "source": {
+                    "url": "https://github.com/desandro/imagesloaded.git",
+                    "type": "git",
+                    "reference": "master"
+                },
+                "dist": {
+                    "url": "https://github.com/desandro/imagesloaded/archive/v4.1.0.zip",
+                    "type": "zip"
+                },
+                "require": {
+                    "composer/installers": "^1.0.20"
+                }
+            }
+        },
+        {
             "type": "vcs",
             "url": "https://github.com/AmazeeLabs/composer-patches"
         },
@@ -150,7 +210,10 @@
         "drupal/admin_language_helper": "dev-amazee#7f60c0f5f1d76c685b89ece0ab3b1dea55d6aa3d",
         "drupal/autotext_widget": "dev-amazee#aa50fea07faaa3ec6613d10a32312d052734d811",
         "drupal/menu_breadcrumb": "dev-amazee#1e4fd13c2889e9000dc91d8cd30cc20b5fb6b2ea",
-        "drupal/contact_storage": "~8.0"
+        "drupal/contact_storage": "~8.0",
+        "enyo/dropzone": "4.3",
+        "desandro/masonry": "4.1",
+        "desandro/imagesloaded": "4.1"
     },
     "conflict": {
         "drupal/drupal": "*"
@@ -182,6 +245,7 @@
             "web/modules/contrib/{$name}": ["type:drupal-module"],
             "web/profiles/contrib/{$name}": ["type:drupal-profile"],
             "web/themes/contrib/{$name}": ["type:drupal-theme"],
+            "web/libraries/{$name}" : ["type:drupal-library"],
             "drush/contrib/{$name}": ["type:drupal-drush"]
         },
         "patches": {