napi-rs Docs

JsValue

JsValue

JsValue represent the JavaScript value in Rust.

JsUndefined

Represent undefined in JavaScript. It can not be casted into Rust value, and no Rust value can be casted into JsUndefined.

The only way to create JsUndefined in Rust is calling Env::get_undefined().

JsNull

Represent null in JavaScript. Like JsUndefined, it can not be casted to Rust value and no Rust value can be casted to it.

The only way to create JsNull in Rust is calling Env::get_null().

JsNumber

f64u32i32i64
FromEnv::create_doubleEnv::create_uint32Env::create_int32Env::create_int64
IntoTryIntoTryIntoTryIntoTryInto
use std::convert::TryInto;
use napi::*;
#[js_function(1)]
fn fib(ctx: CallContext) -> Result<JsNumber> {
let input_number: i64 = ctx.get::<JsNumber>(0)?.try_into()?;
ctx.env.create_int64(fibonacci_native(input_number))
}
#[inline(always)]
fn fibonacci_native(n: i64) -> i64 {
match n {
1 | 2 => 1,
_ => fibonacci_native(n - 1) + fibonacci_native(n - 2),
}
}

JsBoolean

JsBoolean represent boolean value in JavaScript.

Use JsBoolean::get_value() to convert JsBoolean into Rust bool. And Env::get_boolean() to convert Rust bool into JsBoolean.

#[js_function(1)]
fn not(ctx: CallContext) -> Result<JsBoolean> {
let condition: JsBoolean = ctx.get(0)?;
ctx.env.get_boolean(!condition.get_value()?)
}
not(true) // false
not(false) // true

JsString

Represent string value in JavaScript. There 3 types of string encoding you can interactive with N-API: utf8, utf16 and latin1.

There is a article which may help you to understand encoding story in Node.js: https://kevin.burke.dev/kevin/node-js-string-encoding/

Using Env::create_string() you could create JsString from Rust &str. If you want get utf8 value from JsString, you must using the JsString::into_utf8() to get utf8 value explicit.

#[js_function(1)]
fn world(ctx: CallContext) -> Result<JsString> {
// or into_utf16/into_latin1 here
// if you want using into_latin1, you must enable `latin1` feature for `napi-rs`.
let input_string = ctx.get::<JsString>(0)?.into_utf8()?;
let output = format!("{} world!", input_string.as_str()?);
ctx.env.create_string(output.as_str())
}
world('hello') // hello world!

JsBuffer

Represents a Buffer value in Node.js. Passing data between JavaScript and Rust using JsBuffer has a small overhead so you might prefer it over other types.

For example, in some cases, converting a JavaScript string into a Buffer, pass it to Rust as a JsBuffer and cast it as a &[u8] is faster than passing the string directly to Rust. The string implementation in V8 is far more complicated than the ArrayBuffer one, which is what Buffer is implemented by.

#[js_function(1)]
fn set_buffer(ctx: CallContext) -> Result<JsUndefined> {
let buf = &mut ctx.get::<JsBuffer>(0)?.into_value()?; // &mut [u8]
buf[0] = 1;
buf[1] = 2;
ctx.env.get_undefined()
}
setBuffer(Buffer.from([0, 1])) // <Buffer 01 02>

JsSymbol

Represent Symbol value in JavaScript. You can create JsSymbol from &str or JsString

// create from &str
#[js_function(1)]
fn create_symbol(ctx: CallContext) -> Result<JsSymbol> {
let desc = ctx.get::<JsString>(0)?.into_utf8()?;
ctx.env.create_symbol(Some(desc.as_str()?))
}
// create from JsString
#[js_function(1)]
fn create_symbol(ctx: CallContext) -> Result<JsSymbol> {
let desc = ctx.get::<JsString>(0)?;
ctx.env.create_symbol_from_js_string(desc)
}

JsObject

Represent Object value in JavaScript. There are many object related api in JsObject. See document.

#[js_function(1)]
fn set_bar(ctx: CallContext) -> Result<JsUndefined> {
let mut obj = ctx.get::<JsObject>(0)?;
let bar: JsString = obj.get_named_property("bar")?;
let bar_str = bar.into_utf8()?;
obj.set_named_property("bar", ctx.env.create_string_from_std(format!("{} bar", bar_str.as_str()?)))?;
ctx.env.get_undefined()
}
setBar({ bar: 'bar' }) // { bar: "bar bar" }

JsDate

Represent Date object in JavaScript. JavaScript Date objects are described in Section 20.3 of the ECMAScript Language Specification.

JsBigint

Represent Bigint value in JavaScript.

JsExternal

This API allocates a JavaScript value with external data attached to it. This is used to pass external data through JavaScript code, so it can be retrieved later by native code using Env::get_value_external.

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