CallContext

CallContext contains all the information in a JavaScript function call.

Arguments

You can get JavaScript argument via CallContext::get(usize) method:

#[js_function(1)]
fn hey(ctx: CallContext) -> Result<JsUndefined> {
  let arg1: JsString = ctx.get(0)?;
  ctx.env.get_undefined()
}

The argument sequence starts from 0, and can’t be larger than the number passed in the js_function macro. If you pass in an index that is larger than the number in the js_function macro, a JavaScript Error will be thrown.

The type of result must be equal to the actual type of argument, or type error will be thrown.

struct Native {
  value: i32,
}
 
#[js_function(1)]
fn attach_native_object(ctx: CallContext) -> Result<JsUndefined> {
  let count: i32 = ctx.get::<JsNumber>(0)?.try_into()?;
  let mut this: JsObject = ctx.this_unchecked();
  ctx
    .env
    .wrap(&mut this, Native { value: count + 100 })?;
  ctx.env.get_undefined()
}
 
#[js_function(1)]
fn get_native_object(ctx: CallContext) -> Result<JsNumber> {
  let count: i32 = ctx.get::<JsNumber>(0)?.try_into()?;
  let mut this: JsObject = ctx.this_unchecked();
  let native: Native = ctx
    .env
    .unrwap(&mut this)?;
  ctx.env.create_int32(native.value + 1)
}

const obj = {
  attach: attachNativeObject,
  get: getNativeObject,
}
 
obj.attach(100)
obj.get() // 101

Argument length

The number which was passed to js_function is the capability of the arguments Array, the actual arguments length can be retrieved with CallContext::length.

#[js_function(100)]
fn hey(ctx: CallContext) -> Result<JsUndefined> {
  println!("{}", ctx.length);
  ctx.env.get_undefined()
}

hey() // 0
hey({}) // 1

This

You can get this object via CallContext::this or CallContext::this_unchecked. The only different between the two method is CallContext::this will perform a type check, if provided type of return type is not matched with this, InvalidArg error will be thrown.

new target

If the function was called by new operator, you can use CallContext::get_new_target to get the new target of this constructor function.