Custom Js in product view

[HugoFerrer]

Hello,

I want to know the correct form to include custom js to the product view.

I don't understand when to use, data-mage-init and when to use stript tag.

I tried to use the script tag, but i don't see nothing in my front. i flollowed this steps: 

http://magento.stackexchange.com/questions/114327/magento-2-include-js-on-the-product-page/114379

But nothing

I appreciate any help.