Adding external JavaScript file to Magento

[BryanWalters]

How to add external JavaScript file to Magento, so it's code would be included on every frontend page?

[Dmitry_Fedyuk]

• How to add JavaScript / CSS / Less with a custom module: https://mage2.pro/t/34
• How to load JavaScript or stylesheet conditionally depends on backend config settings: https://mage2.pro/t/140
• How to insert some JavaScript to a *.phtml template in the proper way (x-magento-init): https://mage2.pro/t/145
• How to pass parameters from *.phtml to a JavaScript component using x-magento-init: https://mage2.pro/t/146