Module neon::types

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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();

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> {

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:

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


  • bigintnapi-6
    Types for working with JsBigInt.
  • Types and traits for working with binary buffers.
  • Types and traits for working with JavaScript functions.




  • A trait for finalizing values owned by the main JavaScript thread.
  • The trait shared by all JavaScript values.

Type Definitions