Module neon::types

Expand description

Representations of JavaScript’s core builtin types.

Modeling JavaScript Types

All JavaScript values in Neon implement the abstract Value trait, which is the most generic way to work with JavaScript values. Neon provides a number of types that implement this trait, each representing a particular type of JavaScript value.

By convention, JavaScript types in Neon have the prefix Js in their name, such as JsNumber (for the JavaScript number type) or JsFunction (for the JavaScript function type).

Handles and Casts

Access to JavaScript values in Neon works through handles, which ensure the safe interoperation between Rust and the JavaScript garbage collector. This means, for example, a Rust variable that stores a JavaScript string will have the type Handle<JsString> rather than JsString.

Neon types model the JavaScript type hierarchy through the use of casts. The Handle::upcast() method safely converts a handle to a JavaScript value of one type into a handle to a value of its supertype. For example, it’s safe to treat a JsArray as a JsObject, so you can do an “upcast” and it will never fail:

fn as_object(array: Handle<JsArray>) -> Handle<JsObject> {
   let object: Handle<JsObject> = array.upcast();
   object
}

Unlike upcasts, the Handle::downcast() method requires a runtime check to test a value’s type at runtime, so it can fail with a DowncastError:

fn as_array<'a>(
   cx: &mut impl Context<'a>,
   object: Handle<'a, JsObject>
) -> JsResult<'a, JsArray> {
   object.downcast(cx).or_throw(cx)
}

The JavaScript Type Hierarchy

The top of the JavaScript type hierarchy is modeled with the Neon type JsValue. A handle to a JsValue can refer to any JavaScript value. (For TypeScript programmers, this can be thought of as similar to TypeScript’s unknown type.)

From there, the type hierarchy divides into object types and primitive types:

flowchart LR JsValue(JsValue) JsValue-->JsObject(JsObject) click JsValue "./struct.JsValue.html" "JsValue" click JsObject "./struct.JsObject.html" "JsObject" subgraph primitives [Primitive Types] JsBoolean(JsBoolean) JsNumber(JsNumber) JsString(JsString) JsNull(JsNull) JsUndefined(JsUndefined) click JsBoolean "./struct.JsBoolean.html" "JsBoolean" click JsNumber "./struct.JsNumber.html" "JsNumber" click JsString "./struct.JsString.html" "JsString" click JsNull "./struct.JsNull.html" "JsNull" click JsUndefined "./struct.JsUndefined.html" "JsUndefined" end JsValue-->primitives

The top of the object type hierarchy is JsObject. A handle to a JsObject can refer to any JavaScript object.

The primitive types are the built-in JavaScript datatypes that are not object types: JsBoolean, JsNumber, JsString, JsNull, and JsUndefined.

Object Types

The object type hierarchy further divides into a variety of different subtypes:

flowchart LR JsObject(JsObject) click JsObject "./struct.JsObject.html" "JsObject" subgraph objects [Standard Object Types] JsFunction(JsFunction) JsArray(JsArray) JsDate(JsDate) JsError(JsError) click JsFunction "./struct.JsFunction.html" "JsFunction" click JsArray "./struct.JsArray.html" "JsArray" click JsDate "./struct.JsDate.html" "JsDate" click JsError "./struct.JsError.html" "JsError" end subgraph typedarrays [Typed Arrays] JsBuffer(JsBuffer) JsArrayBuffer(JsArrayBuffer) JsTypedArray("JsTypedArray<T>") click JsBuffer "./struct.JsBuffer.html" "JsBuffer" click JsArrayBuffer "./struct.JsArrayBuffer.html" "JsArrayBuffer" click JsTypedArray "./struct.JsTypedArray.html" "JsTypedArray" end subgraph custom [Custom Types] JsBox(JsBox) click JsBox "./struct.JsBox.html" "JsBox" end JsObject-->objects JsObject-->typedarrays JsObject-->custom

These include several categories of object types:

Standard object types: JsFunction, JsArray, JsDate, and JsError.
Typed arrays: JsBuffer, JsArrayBuffer, and JsTypedArray<T>.
Custom types: JsBox, a special Neon type that allows the creation of custom objects that own Rust data structures.

All object types implement the Object trait, which allows getting and setting properties of an object.

Modules

buffer

Types and traits for working with binary buffers.

function

Types and traits for working with JavaScript functions.

Structs

DateErrornapi-5

The Error struct for a Date

Deferred

Deferred is a handle that can be used to resolve or reject a JsPromise

JsArray

A JavaScript array object, i.e. a value for which Array.isArray would return true.

JsArrayBuffer

The standard JS ArrayBuffer type.

JsBoolean

A JavaScript boolean primitive value.

JsBox

A smart pointer for Rust data managed by the JavaScript engine.

JsBuffer

The Node Buffer type.

JsDatenapi-5

A JavaScript Date object

JsError

A JS Error object.

JsFunction

A JavaScript function object.

JsFuturenapi-5 and futures

A Future created from a JsPromise.

JsNull

The JavaScript null value.

JsNumber

A JavaScript number value.

JsObject

A JavaScript object.

JsPromise

The JavaScript Promise value.

JsString

A JavaScript string primitive value.

JsTypedArray

The family of JS typed array types.

JsUndefined

The JavaScript undefined value.

JsValue

A JavaScript value of any type.

StringOverflow

An error produced when constructing a string that exceeds the JS engine’s maximum string size.

Enums

DateErrorKindnapi-5

The error kinds corresponding to DateError

Traits

Finalize

Finalize is executed on the main JavaScript thread and executed immediately before garbage collection. Values contained by a JsBox must implement Finalize.

Value

The trait shared by all JavaScript values.