How I mentioned in my previous post How to add ReactJS in Drupal 8 with Composer, Composer is a fundamental part of Drupal 8 development workflow.
Like any open source projects, contributions are the core of Drupal, and those contributions are included in Drupal modules and themes and event Drupal core via Patches.
1. Drupal Installation.
In this example I will assume that did you install Drupal using *Drupal Composer* project, using an instruction similar to the following: