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Neon provides built-in mechanisims for accessing the arguments object.

Arguments can be passed from JS to Rust and be of any type. It is useful to assert that certain values are certain types.

Calling Functions by Indexes#

We first start by defining a function and exporting it by the name of sayHi:

fn say_hi(mut cx: FunctionContext) {}
register_module!(mut m, {
m.export_function("sayHi", say_hi)

The following code takes the first argument passed to the sayHi function and throws if it cannot be cast to a function

fn say_hi(mut cx: FunctionContext) -> JsResult<JsFunction> {
let arg0 = cx.argument::<JsFunction>(0)?;
// --snip--

Asserting Argument Types#

pub fn foo(mut cx: FunctionContext) -> JsResult<JsUndefined> {

Now in our ./lib/index.js:

const { foo } = require('../native');
foo(); // fails
foo(12); // fails
foo('foobar'); // fails
foo('foobar', 12); // passes!

Getting the Value of an Argument#

fn add1(mut cx: FunctionContext) -> JsResult<JsNumber> {
// Attempt to cast the first argument to a JsNumber. Then
// get the value if cast is successul
let x = cx.argument::<JsNumber>(0)?.value();
Ok(cx.number(x + 1.0))
register_module!(mut m, {
m.export_function("add1", add1)

Getting the Number of Arguments#

This is a simple example of getting the length of arguments

pub fn get_args_len(mut cx: FunctionContext) -> JsResult<JsNumber> {
let args_length = cx.len();
println!("{}", args_length);
register_module!(mut m, {
m.export_function("getArgsLen", get_args_len)

Now in our ./lib/index.js we add the following:

// ./lib/index.js
const { getArgsLen } = require('../native');
getArgsLen(); // 0
getArgsLen(1); // 1
getArgsLen(1, 'foobar'); // 2

Optional Arguments#

Produces the ith argument, or None if i is greater than or equal to self.len().

pub fn args_opt(mut cx: FunctionContext) -> JsResult<JsNumber> {
match cx.argument_opt(0) {
Some(arg) => {
// Throw if the argument exist and it cannot be downcasted
// to a number
let num = arg.downcast::<JsNumber>().or_throw(&mut cx)?.value();
println!"The 0th argument is {}", num);
None => panic!("0th argument does not exist, out of bounds!")

Default Values#

Handling default values is similar to handling Optional Arguments:

// --snip--
pub fn default_args(mut cx: FunctionContext) -> JsResult<JsUndefined> {
let age = match cx.argument_opt(0) {
Some(arg) => arg.downcast::<JsNumber>().or_throw(&mut cx)?.value(),
// Default to 12 if no value is given
None => 12 as f64
let name = match cx.argument_opt(1) {
Some(arg) => arg.downcast::<JsString>().or_throw(&mut cx)?.value(),
// Default to 12 if no value is given
None => "John Doe".to_string()
println!("i am {} years old and my name is {}", age, name);

Here's how we'd call those functions:

// ./lib/index.js
const { defaultArgs } = require('../native');
defaultArgs(); // i am 12 years old and my name is John Doe
defaultArgs(22); // i am 22 years old and my name is John Doe
defaultArgs(22, 'Jane Doe'); // i am 22 years old and my name is Jane Doe