# src/lib.rs

`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`](#a1-wasm_bindgen-functions)
2. [Crate imports](#a2-crate-imports)
3. [`wee_alloc` optional dependecy](#a3-wee_alloc-optional-dependecy)
	- [What is `wee_alloc`?](#what-is-wee_alloc)

---

We'll start with the most important part of `lib.rs` -- the two `#[wasm_bindgen]` functions. In many cases, this is the only part of `lib.rs` you will need to modify.

## 1. `#[wasm_bindgen]` functions

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

```rust
#[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.

```rust
#[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! If you are curious about the rest, read on.

## 2. Crate imports

```rust
extern crate cfg_if;
extern crate wasm_bindgen;
```

In `Cargo.toml`, we included the crates `cfg_if` and `wasm_bindgen` as project dependencies.

Here, we explicitly declare that these crates will be used in `lib.rs`.

```rust
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:

```rust
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`.


```rust
use cfg_if::cfg_if;
```

`use` allows us to conveniently refer to parts of a crate or module. For example, suppose the crate `cfg_if` contains a function `func`. It is always possible to call this function directly by writing `cfg_if::func()`. However, this is often tedious to write. If we first specify `use cfg_if::func;`, then `func` can be called by just writing `func()` instead.

With this in mind, this `use` allows us to call the macro `cfg_if!` inside the crate `cfg_if` without writing `cfg_if::cfg_if!`. We use `cfg_if!` to configure `wee_alloc`, which we will talk more about in a [separate section](./wee_alloc.md):

```rust
cfg_if! {
    if #[cfg(feature = "wee_alloc")] {
        extern crate wee_alloc;
        #[global_allocator]
        static ALLOC: wee_alloc::WeeAlloc = wee_alloc::WeeAlloc::INIT;
    }
}
```

We immediately notice that `cfg_if!` is a macro because it ends in `!`, similarly to other Rust macros such as `println!` and `vec!`. A macro is directly replaced by other code during compile time.

During compile time, `cfg_if!` evaluates the `if` statement. This tests whether the feature `wee_alloc` is present in the `[features]` section of `Cargo.toml` (among other possible ways to set it).

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

```rust
use wasm_bindgen::prelude::*;
```

Many modules contain a prelude, a list of things that should be automatically imported. 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.