Managing dependencies with Composer Manager

Prerequisites:

Drupal 8
Composer

Use Case:

You want to add dependencies to your custom module using composer update at the root of your site directory.
Updating Drupal core removes any custom dependencies added to the root composer.json, potentially breaking any modules that require them.

Quick run-down:

First, download composer manager to your site’s modules directory and run the init.php command to register the ‘drupal-update’ command. This command is what will look through your module’s composer.json files and update the main composer.json file at your site’s docroot.

cd mysiteroot
drush dl composer_manager
php modules/composer_manager/scripts/init.php
composer drupal-update

In your sites directory, add a composer.json file and make sure you have both the name and requirements:

{
  "name": "drupal/mymodule",
  "require": {
    "mailchimp/mailchimp": "2.0.6"
  }
}

Now, run composer drupal-update again and you should see your dependencies in the vendor directory.