What's the idiomatic way to type js closures in d.ts?

[hillin]

E.g. in the example of Receiving JavaScript Closures in Exported Rust Functions:

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);
        }
    }
}

I'd like to type the f parameter of the each function in the d.ts file, supposedly like this:

each(f: (value: number) => void);

[daxpedda]

This is currently not supported.

Duplicate of #3107.

[Liamolucko]

This is currently not supported.

Yes it is, you can make a custom subclass of js_sys::Function and give it the appropriate typescript_type:

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen(extends = js_sys::Function, typescript_type = "(value: number) => void")]
    pub type NumberFunc;
}

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

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

The same solution works for #3107. (Well, not perfectly, the OP there asked for an attribute on the function rather than having to declare a type, but more or less.)