Support optional fields in struct reflect in typescript

[clearloop]

Motivation

Support optional fields in struct reflect in typescript, for example:

#[wasn_bindgen]
pub struct Ping {
    pong: Option<bool>,
}

in .d.ts

export class Ping {
    pong?: bool;
}

Proposed Solution

Struct generation in wasm_bindgen separate fields into getters and setters, and the ret_ty for getter or args for setter which decide the final field type out are embedded in JsBuilder, we can't checkout if some fields' types are optional, but...optional fields generate some special docs like @returns{{{}}} and @params{{{}}} we can trace.

If it is appropriate to render fields' types with checking if its doc contains .. | undefined ?

For example, we can change this line

wasm-bindgen/crates/cli-support/src/js/mod.rs

Line 3070 in f507a2a

*ty = ret_ty.to_string();

if docs.contains("undefined") {
    *ty = "?".to_string() + ret_ty
} else {
    *ty = ret_ty.to_string()
}

and this line

wasm-bindgen/crates/cli-support/src/js/mod.rs

Line 703 in f507a2a

ts_dst.push_str(ty);

if &ty[0..1] == "?" {
    ts_dst.push_str("?: ");
    ts_dst.push_str(&ty[1..]);
} else {
    ts_dst.push_str(": ");
    ts_dst.push_str(&ty);
}

Alternatives

I think the solution above is quite a mess, but I can't figure out a better one.

[Pauan]

I like the idea of generating optional fields... however, I would suggest that whatever tool you are using should work with | undefined, because from JS's perspective there isn't much difference, and many TypeScript programs will use | undefined.

[clearloop]

The | undefined checking just applies to setter and getter in .d.ts, if this solution can make a pr, I'd like to do it, or any other way can add optional fields definition to Typescript...really need this.

btw, @Pauan, would you mind to have a look at #1986, I just wrote a pr about #[wasm_bindgen(plain_object)] yesterday.

The words in your avatar are really awesome! “花鳥" means flowers and birds in Chinese and "旅鳥" means a bird keep travelling!

[alexcrichton]

Seems like a reasonable feature to me to support as well! A PR would be most welcome to implement this