My first plugin. Please help me understand simple things

[romanusname]

Hello! I am creating the first plugin for swc. That's why I started learning Rust.
Help me understand. The documentation is hard for me.

1. https://rustdoc.swc.rs/swc_ecma_visit/trait.VisitMut.html

How can I understand what these methods return?

    fn visit_mut_bin_expr (&mut self, e: &mut BinExpr ){
        e.visit_mut_children_with(self);
        if e.op == op!("===") {
            e.left = Box::new(Ident::new("kdy1".into(), e.left.span()).into());
        }
    }

What does e.left do?
what is Box?

2. How does the test work?

test!(
    Default::default(),
    |_| as_folder(TransformVisitor),
    boo,
    r#"foo === bar;"#,
    r#"kdy1 === bar;"#
);

Why are 3 lines needed? Will I need to change them?

    Default::default(),
    |_| as_folder(TransformVisitor),
    boo,

What do these 2 lines do? How do they test? what is r#?

    r#"foo === bar;"#,
    r#"kdy1 === bar;"#

I would be happy to receive answers to at least some of the questions. I really want to write my plugin

[romanusname]

I understand. https://play.swc.rs/ helps