Expand description

This is an alternative native integration of Serde with wasm-bindgen.


This library was created to address rustwasm/wasm-bindgen#1258 and provide a native Serde integration for wasm-bindgen to directly convert values between JavaScript and Rust (compiled to WebAssembly).

The primary difference with the built-in implementation is that it leverages direct APIs for JavaScript value manipulation instead of passing data in a JSON format. This allows it to support more types while producing a much leaner Wasm binary. In particular, it saved 26.6KB when comparing size-optimised and Brotli-compressed benchmarks with a stripped debug information.

Performance-wise the library is currently comparable with the original. Specific numbers vary a lot between the engines and used data types and, according to benchmarks, range from 1.6x regression in worst cases to 3.3x improvement in best cases. Your mileage might vary.

These numbers are currently mostly saturated by the overhead of frequent JavaScript <-> Wasm and JavaScript <-> C++ calls. These calls are used for sharing JavaScript values with the Rust side as well as encoding/decoding UTF-8 strings, and will go away in the future when reference types proposal lands natively in Wasm.


To pass a Rust value to JavaScript, use:

use wasm_bindgen::prelude::*;

pub fn pass_value_to_js() -> Result<(), JsValue> {
	let some_supported_rust_value = ("Hello, world!", 42);
	let js_value = serde_wasm_bindgen::to_value(&some_supported_rust_value)?;
	// ...

To retrieve a value from JavaScript:

use wasm_bindgen::prelude::*;

pub fn get_value_from_js(value: JsValue) -> Result<(), JsValue> {
	let value: (String, i32) = serde_wasm_bindgen::from_value(value)?;
	// ...

Supported types

Note: this library is not strictly compatible with either serde_json or, correspondingly, JsValue::from_serde / JsValue::into_serde, by default, for better compatibility with common JavaScript idioms and representations. If you need compatibility with them, or you want to use JSON.stringify on the result without data loss, use Serializer::json_compatible() as serializer.

Supported types and values for the deserialization:

  • () from undefined and null.
  • Option from any value will map undefined or null to None and any other value to Some(...).
  • bool from a JavaScript boolean (false and true).
  • Rust integer (u8/i8/…/u128/i128) from a safe JavaScript integer (as matched by Number.isSafeInteger).
  • Rust floating number (f32/f64) from any JavaScript number.
  • char from a JavaScript string containing a single codepoint.
  • String from any JavaScript string.
  • Rust map (HashMap, BTreeMap, …) from any JavaScript iterable producing [key, value] pairs (including but not limited to ES2015 Map).

    One exception being internally tagged and untagged enums. These representations currently do not support deserializing map-like iterables. They only support deserialization from Object due to their special treatment in serde.

    This restriction may be lifted at some point in the future if a serde(with = ...) attribute can define the expected Javascript representation of the variant, or if serde-rs/serde#1183 gets resolved.

  • HashMap<String, _> from any plain JavaScript object ({ key1: value1, ... }).
  • Rust sequence (tuple, Vec, HashSet, …) from any JavaScript iterable (including but not limited to Array, ES2015 Set, etc.).
  • Rust byte buffer (see serde_bytes) from JavaScript ArrayBuffer or Uint8Array.
  • Typed Rust structure from any plain JavaScript object ({ key1: value1, ... }).
  • Rust enum from either a string ("Variant") or a plain object. Specific representation is controlled by #[serde(...)] attributes and should be compatible with serde-json.

Serialization is compatible with the deserialization, but it’s limited to a single representation, so it chooses:

  • undefined for () or None (can be configured to use null via serialize_missing_as_null(true)).
  • ES2015 Map for Rust maps (can be configured to use plain objects via serialize_maps_as_objects(true)).
  • Array for any Rust sequences.
  • Uint8Array for byte buffers.
  • Plain JavaScript object for typed Rust structures.
  • JavaScript number type for any Rust numeric types. u64 and i64 can be configured to use BigInt instead via serialize_large_number_types_as_bigints(true).


Licensed under the MIT license. See the LICENSE file for details.


A newtype that allows using any JsValue as a serde::Deserializer.

A newtype that represents Serde errors as JavaScript exceptions.

A serde::Serializer that converts supported Rust values into a JsValue.


Converts JsValue into a Rust type.

Converts a Rust value into a JsValue.