wasm-bindgen/guide/src/reference/receiving-js-closures-in-rust.md

Receiving JavaScript Closures in Exported Rust Functions

You can use the js-sys crate to access JavaScript's Function type, and invoke that function via Function.prototype.apply and Function.prototype.call.

For example, we can wrap a Vec<u32> in a new type, export it to JavaScript, and invoke a JavaScript closure on each member of the Vec:

extern crate js_sys;
extern crate wasm_bindgen;

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub struct VecU32 {
    xs: Vec<u32>,
}

#[wasm_bindgen]
impl VecU32 {
    pub fn each(&self, f: &js_sys::Function) {
        let this = JsValue::NULL;
        for x in &self.xs {
            let x = JsValue::from(x);
            let _ = f.call1(&this, &x);
        }
    }
}

Since Rust has no function overloading, the call# method also requires a number representing the amount of arguments passed to the JavaScript closure.