Neon 1.0.0 Migration Guide

Note: This migration guide assumes a project is using Neon 0.10 without Node-API backend. If using an older version or the legacy backend, see the previous migration guide.

The Neon 1.0 has stabilized and brings a more consistent and ergonomic API. There are a few (minor) breaking changes and this guide will help walk through them!

Removed Traits

A few traits have been removed because they were either redundant or only used for features that no longer exist.

Managed

The Managed trait marked values that were managed by the JavaScript VM. It was redundant with the Value trait. Trait bounds referencing Managed may be either removed or replaced with Value.

Before

fn example<V>(h: Handle<V>)
where
    V: Managed,
{
}

After

fn example<V>(h: Handle<V>)
where
    V: Value,
{
}

CallContext, This, and T in JsFunction<T>

The This trait marked values for cx.this() in JsFunction. However, it was not type checked and could result in a panic at runtime. Instead, cx.this() always returns a JsValue. Since, JsFunction's T parameter had a default, in many cases no changes are necessary. In some cases, the T parameter will need to be removed and a downcast added.

Before

// `CallContext<JsObject>` is equivalent to `FunctionContext`
fn example(mut cx: CallContext<JsObject>) -> JsResult<JsUndefined> {
    let a = cx.this().get::<JsValue, _, _>(&mut cx, "a")?;

    Ok(cx.undefined())
}

After

fn example(mut cx: FunctionContext) -> JsResult<JsUndefined> {
    let a = cx.this::<JsObject>()?.get::<JsValue, _, _>(&mut cx, "a")?;

    Ok(cx.undefined())
}

JsResultExt

The JsResultExt trait provided a .or_throw(&mut cx) to allow converting a Rust error into a JavaScript exception. However, it required T: Value. It has been replaced with a more generic ResultExt trait. Most usages only require replacing JsResultExt with ResultExt. In some cases, an additional T: Value bound will need to be added or removed.

Before

use neon::result::JsResultExt;

After

use neon::result::ResultExt;

usize indexes and lengths

Neon inconsistently used u32, usize, and sometimes even i32 for indexes and lengths. For consistency with Rust, usize is used everywhere. Update explicit types to use usize and remove type casting. Implicit types do not need to be updated.

Before

fn example(mut cx: FunctionContext) -> JsResult<JsUndefined> {
    let arr = cx.empty_array();
    let msg = cx.string("Hello!");

    arr.set(&mut cx, 0u32, msg)?;

    Ok(cx.undefined())
}

After

fn example(mut cx: FunctionContext) -> JsResult<JsUndefined> {
    let arr = cx.empty_array();
    let msg = cx.string("Hello!");

    arr.set(&mut cx, 0usize, msg)?;

    Ok(cx.undefined())
}

Feature Flags

Neon 0.10 made extensive use of feature flags for features that had not been stabilized (e.g., try-catch-api, channel-api). All features have been stabilized and the feature flags removed. Resolve by removing these features from the project's Cargo.toml.

Two feature flags are still exist: napi-N for the Node-API version and futures for compatibility between Rust Future and JavaScript Promise.