How to add a javascript library to composer.json in drupal 8

It is seen as a bad practice to add complete libraries to your theme in drupal 8. You should add these as packages to your composer file. 

"repositories": [
    {
        "type": "composer",
        "url": "https://packages.drupal.org/8"
    },
    {
        "type": "package",
        "package": {
          "name": "jackmoore/colorbox",
          "version": "1.6.4",
          "type": "drupal-library",
          "dist": {
            "url": "https://github.com/jackmoore/colorbox/archive/1.6.4.zip",
            "type": "zip"
          }
        }
    }
],