Imagine you have a var variable containing: <div>I'm happy</div>.
On index.twig, {{ var }} will render <div>I'm happy</div>.
On index.html.twig, {{ var }} will render <div>I'm happy<div>
On index.js.twig, {{ var }} will render \x3Cdiv\x3EI\x27m\x20happy\x3Cdiv\x3E
And so on.
Always use the right extension to avoid any XSS vulnerability, and
always use |raw wisely because it overlaps this extension’s implicit protection.