Support for Option in .d.ts

[henriiik]

Currently, if you return an Option from a function in rust, the generated .d.ts file does not reflect that the function can return undefined. I am willing to help out if this is a desired change. 🙂

For example if you have the following rust,

extern crate wasm_bindgen;
use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub fn some_number() -> usize {
    1
}

#[wasm_bindgen]
pub fn some_optional_number() -> Option<usize> {
    None
}

you get the following d.ts,

/* tslint:disable */
export function some_number(): number;

export function some_optional_number(): number;

But the actual return value has the type number | undefined. if you run typescript in strict mode, it will force you to check for undefined, just like rust.

Or you could go even further and define a new type in typescript, ex:

type Option<T> = T | undefined;

export function some_optional_number(): Option<number>;

Would you be willing to merge a change like this if I send a PR? 🙂

[alexcrichton]

Thanks for the report! This is related to #922 where we unfortunately don't test typescript much on CI, but I think this is still imminently fixable and it can surely be fixed through a PR!

I think the Option<T> idiom may not be necessary though? It looks like typescript supports directly returning a type that looks like number | undefined, so we can probably inline that definition at each function call.

[henriiik]

Sounds good! I'll send a PR, probably wont be able to get to it until the weekend though. :)

[henriiik]

this is fixed. :)