lib.rs is the template's main source file. The name lib.rs commonly implies that this Rust project will be compiled as a library.

It contains three key parts:

1. #[wasm_bindgen] functions
2. Crate imports
3. wee_alloc optional dependecy
   • What is wee_alloc?

We'll start with the most important part of lib.rs -- the two #[wasm_bindgen] functions (which you can find at the bottom of the file). In many cases, this is the only part of lib.rs you will need to modify.

To expose functionality from the wasm-bindgen crate more conveniently we can use the use keyword. use allows us to conveniently refer to parts of a crate or module. You can learn more about how Rust lets you write modular code in this chapter of the book.

# #![allow(unused_variables)]
#fn main() {
use wasm_bindgen::prelude::*;
#}

Many crates contain a prelude, a list of things that are convenient to import all at once. This allows common features of the module to be conveniently accessed without a lengthy prefix. For example, in this file we can use #[wasm_bindgen] only because it is brought into scope by the prelude.

The asterisk at the end of this use indicates that everything inside the module wasm_bindgen::prelude (i.e. the module prelude inside the crate wasm_bindgen) can be referred to without prefixing it with wasm_bindgen::prelude.

For example, #[wasm_bindgen] could also be written as #[wasm_bindgen::prelude::wasm_bindgen], although this is not recommended.

The #[wasm_bindgen] attribute indicates that the function below it will be accessible both in JavaScript and Rust.

# #![allow(unused_variables)]
#fn main() {
#[wasm_bindgen]
extern {
    fn alert(s: &str);
}
#}

The extern block imports the external JavaScript function alert into Rust. This declaration is required to call alert from Rust. By declaring it in this way, wasm-bindgen will create JavaScript stubs for alert which allow us to pass strings back and forth between Rust and JavaScript.

We can see that the alert function requires a single parameter s of type &str, a string. In Rust, any string literal such as "Hello, test-wasm!" is of type &str. So, alert could be called by writing alert("Hello, test-wasm!");.

We knew to declare alert in this way because it is how we would call alert in JavaScript -- by passing it a string argument.

# #![allow(unused_variables)]
#fn main() {
#[wasm_bindgen]
pub fn greet() {
    alert("Hello, test-wasm!");
}
#}

If we were to write the greet function without the #[wasm_bindgen] attribute, then greet would not be easily accessible within JavaScript. Furthermore, we wouldn't be able to natively convert certain types such as &str between JavaScript and Rust. So, both the #[wasm_bindgen] attribute and the prior import of alert allow greet to be called from JavaScript.

This is all you need to know to interface with JavaScript, at least to start! You can learn a bunch more by reading the wasm-bindgen documentation!

If you are curious about the rest, read on.

# #![allow(unused_variables)]
#fn main() {
mod utils;
#}

This statement declares a new module named utils that is defined by the contents of utils.rs. Equivalently, we could place the contents of utils.rs inside the utils declaration, replacing the line with:

# #![allow(unused_variables)]
#fn main() {
mod utils {
    // contents of utils.rs
}
#}

Either way, the contents of utils.rs define a single public function set_panic_hook. Because we are placing it inside the utils module, we will be able to call the function directly by writing utils::set_panic_hook(). We will discuss how and why to use this function in src/utils.rs.

# #![allow(unused_variables)]
#fn main() {
    // When the `wee_alloc` feature is enabled, use `wee_alloc` as the global
    // allocator.
    if #[cfg(feature = "wee_alloc")] {
        #[global_allocator]	static ALLOC: wee_alloc::WeeAlloc = wee_alloc::WeeAlloc::INIT;
#}

At compile time this will test if the wee_alloc feature is enabled for this compilation. If it's enabled we'll configure a global allocator (according to wee_alloc's docs), otherwise it'll compile to nothing.

As we saw earlier, the default vector in [features] only contains "console_error_panic_hook" and not "wee_alloc". So, in this case, this block will be replaced by no code at all, and hence the default memory allocator will be used instead of wee_alloc.