Migration guide entry for Object::get_value() and Object::get_opt()

[dherman]

This PR adds information to the migration guide for Object::get() use cases that encode optional or union types.

[thekingofcity]

I can't find an issue for the original MIGRATION_GUIDE_0.10.md so I'm commenting here. Thanks for this wonderful guide and this helps me keep a simple js_log function up to date.

---

pub fn js_log<'a, C: Context<'a>>(cx: &mut C, s: String) -> NeonResult<()> {
    let global = cx.global();
    let console = global.get(cx, "console")?.downcast_or_throw::<JsObject, _>(cx)?;
    let log = console.get(cx, "log")?.downcast_or_throw::<JsFunction, _>(cx)?;
    let null = cx.null();

    let args: Vec<Handle<JsString>> = vec![cx.string(s)];
    log.call(cx, null, args)?;

    Ok(())
}

version: 0.9.0

---

pub fn js_log<'a, C: Context<'a>>(cx: &mut C, s: String) -> NeonResult<()> {
    let global = cx.global();
    let console = global.get(cx, "console")?.downcast_or_throw::<JsObject, _>(cx)?;
    let log = console.get(cx, "log")?.downcast_or_throw::<JsFunction, _>(cx)?;

    log.call_with(cx).arg(cx.string(s)).apply::<JsUndefined, C>(cx)?;

    Ok(())
}

version 0.10.0-alpha2

---

pub fn js_log<'a, C: Context<'a>>(cx: &mut C, s: String) -> NeonResult<()> {
    let global = cx.global();
    let console = global.get::<JsObject, _, _>(cx, "console")?;
    let log = console.get::<JsFunction, _, _>(cx, "log")?;

    log.call_with(cx).arg(cx.string(s)).apply::<JsUndefined, C>(cx)?;

    Ok(())
}

version 0.10.0

---

BTW, console.log should be a very important feature especially during debugging, but I can't find an example about this in doc or code. Maybe it would be great to have it somewhere in the repo?

[kjvalencik]

Thanks for the feedback @thekingofcity! FYI, here's the PR for the migration guide.

console.log is an excellent suggestion! It would be nice to have a simple example like that. There's a PR for a more complex example: (connecting Rust's log logging facade to the debug node module): neon-bindings/examples#84.

You can simplify the js_log example a little bit by using CallOptions::exec instead of apply. It discards the return value which avoids the need to provide the type.;

 pub fn js_log<'a, C: Context<'a>>(cx: &mut C, s: String) -> NeonResult<()> {
     let global = cx.global();
     let console = global.get::<JsObject, _, _>(cx, "console")?;
     let log = console.get::<JsFunction, _, _>(cx, "log")?;

-    log.call_with(cx).arg(cx.string(s)).apply::<JsUndefined, C>(cx)?;
+    log.call_with(cx).arg(cx.string(s)).exec(cx)?;

     Ok(())
 }

It also looks like this could benefit from a recent proposal for Object::call_method. #873

[thekingofcity]

Would be great to see that logging example merged! Of course an out-of-box crate or snippets will be even more user-friendly. I also changed apply to exec, thanks for the suggestion. 😁

PS I find a migration guide for 0.2 in the wiki, but the 0.10 guide is in the repo. Should we place them in one place?