函数

在 NAPI-RS 中定义一个 JavaScript function 非常简单，只需要一个普通的 Rust fn：

lib.rs

#[napi]
fn sum(a: u32, b: u32) -> u32 {
  a + b
}

最重要的是，你需要记住 NAPI-RS fn 并不支持所有的 Rust 类型，下面的表格说明了在 fn 参数和返回类型里，JavaScript 类型和 Rust 类型的映射关系：

参数

Rust Type	JavaScript Type
`u32`	`number`
`i32`	`number`
`i64`	`number`
`f64`	`number`
`bool`	`boolean`
`String`	`string`
`Latin1String`	`string`
`UTF16String`	`string`
`#[napi(object)] struct`	`Object`
`& struct` or `&mut struct`	Class instance
`serde_json::Map`	`Object`
`serde_json::Value`	`unknown`
`std::collections::HashMap`	`Object`
`Array`	`unknown[]`
`Vec<T>` T must be types in this table	T[]
`Buffer`	`Buffer`
`External`	`External` (opens in a new tab)
`Null`	`null`
`Undefined` / `()`	`undefined`
`Option<T>`	`T or null`
`Fn(Arg) ->T` `Arg` and `T` must be types in this table	`(arg: Arg) => T`
`Promise<T>`	`Promise<T>`
`AbortSignal`	`AbortSignal` (opens in a new tab)
`JsSymbol`	`Symbol`
`Int8Array` / `Uint8Array` / `Int16Array`...	`TypedArray`
`BigInt`	`BigInt`

Rust Type	JavaScript Type
`u32`	`number`
`i32`	`number`
`i64`	`number`
`f64`	`number`
`bool`	`boolean`
`String`	`string`
`Latin1String`	`string`
`UTF16String`	`string`
`#[napi(object)] struct`	`Object`
`#[napi] struct`	Class
`serde_json::Map`	`Object`
`serde_json::Value`	`unknown`
`std::collections::HashMap`	`Object`
`Array`	`unknown[]`
`Vec<T>` T must be types in this table	`T[]`
`Buffer`	`Buffer`
`External`	`External`
`Null`	`null`
`Undefined` / `()`	`undefined`
`Option<T>`	`T` or `null`
`AsyncTask<Task<JsValue = T>>`	`Promise<T>`
`JsSymbol`	`Symbol`
`Int8Array` / `Uint8Array` / `Int16Array`...	`TypedArray`
`BigInt`	`BigInt`
`i64n`	`BigInt`
`i128`	`BigInt`
`u128`	`BigInt`