How to pass function callbacks?

[ctaggart]

I am trying to figure out how to use function callbacks. In this example, I'm simply creating a numeric array and trying to filter for just the even numbers. Unfortunately, the returned array is always empty. Any idea how to make this work? Here is what I last tried:

extern crate chakracore as js;

use std::io::prelude::*;
use std::fs::File;
use js::Property;

fn main() {
    let runtime = js::Runtime::new().unwrap();
    let context = js::Context::new(&runtime).unwrap();
    let guard = context.make_current().unwrap();


    // create a JavaScript Array
    let length = 10;
    let array = js::value::Array::new(&guard, length);
    for i in 0..length {
        array.set_index(&guard, i, &js::value::Number::new(&guard, i as i32));
    }
    println!("array: {:?}", array.len(&guard));


    // try filtering it
    let is_even = js::value::Function::new(&guard, Box::new(|guard, info| {
        //        let i = info.arguments[0].to_integer(guard);
        //        println!("i: {:?}", i);
        //        Ok(js::value::Boolean::new(guard, i % 2 == 0).into())
        println!("is_even");
        Ok(js::value::Boolean::new(guard, true).into())
    }));

    let fn_filter = array.get(&guard, &Property::new(&guard, "filter")).into_function().unwrap();

    let filtered_array = fn_filter.call(&guard, &[&is_even]).unwrap(); //.into_array().unwrap();
    println!("filtered_array: {:?}", filtered_array);

    //    for v in filtered_array.iter(&guard){
    //        println!("filtered v: {:?}", v);
    //    }

}

[darfink]

You are calling the filter method without any context (i.e this is not set to an instance).
Which roughly corresponds to this javascript:

let array = [];
for (var i = 0; i < 10; i++) {
  array.push(i);
}
let is_even = (x) => x % 2 == 0;
let fn_filter = array.filter;
fn_filter(is_even);

Therefore your solution is to call fn_filter with the array instance:

let filtered_array = fn_filter.call(&guard, &[&is_even]).unwrap();
// --->
let filtered_array = fn_filter.call_with_this(&guard, &array, &[&is_even]).unwrap();

[ctaggart]

Thank you for the clear explanation. It works great.

Cleaned up & works:

extern crate chakracore as js;

use js::Property;

fn main() {
    let runtime = js::Runtime::new().unwrap();
    let context = js::Context::new(&runtime).unwrap();
    let guard = context.make_current().unwrap();

    // create a JavaScript Array
    let length = 10;
    let array = js::value::Array::new(&guard, length);
    for i in 0..length {
        array.set_index(&guard, i, &js::value::Number::new(&guard, i as i32));
    }

    // try filtering it
    let is_even = js::value::Function::new(&guard, Box::new(|guard, info| {
        let i = info.arguments[0].to_integer(guard);
        Ok(js::value::Boolean::new(guard, i % 2 == 0).into())
    }));

    let fn_filter = array.get(&guard, &Property::new(&guard, "filter")).into_function().unwrap();

    let filtered_array = fn_filter.call_with_this(&guard, &array, &[&is_even]);
    println!("filtered_array: {:?}", filtered_array);
}

prints as expected:

filtered_array: Ok(Value(Array: 0,2,4,6,8))