created_at2019-05-16 17:15:02.792426
updated_at2024-02-27 21:32:21.678632
descriptionNative Serde adapter for wasm-bindgen
Ingvar Stepanyan (RReverser)


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This is a native integration of Serde with wasm-bindgen. It allows to convert Rust data types into native JavaScript types and vice versa.

Initially this library was created while working for @Cloudflare as an alternative implementation to the JSON-based Serde support built into the wasm-bindgen but, nowadays serde-wasm-bindgen is the officially preferred approach. It provides much smaller code size overhead than JSON, and, in most common cases, provides much faster serialization/deserialization as well.


Copied almost verbatim from the wasm-bindgen guide:

Add dependencies

To use serde-wasm-bindgen, you first have to add it as a dependency in your Cargo.toml. You also need the serde crate, with the derive feature enabled, to allow your types to be serialized and deserialized with Serde.

serde = { version = "1.0", features = ["derive"] }
serde-wasm-bindgen = "0.4"

Derive the Serialize and Deserialize Traits

Add #[derive(Serialize, Deserialize)] to your type. All of your type members must also be supported by Serde, i.e. their types must also implement the Serialize and Deserialize traits.

Note that you don't need to use the #[wasm_bindgen] macro.

use serde::{Serialize, Deserialize};

#[derive(Serialize, Deserialize)]
pub struct Example {
    pub field1: HashMap<u32, String>,
    pub field2: Vec<Vec<f32>>,
    pub field3: [f32; 4],

Send it to JavaScript with serde_wasm_bindgen::to_value

pub fn send_example_to_js() -> Result<JsValue, JsValue> {
    let mut field1 = HashMap::new();
    field1.insert(0, String::from("ex"));

    let example = Example {
        field2: vec![vec![1., 2.], vec![3., 4.]],
        field3: [1., 2., 3., 4.]


Receive it from JavaScript with serde_wasm_bindgen::from_value

pub fn receive_example_from_js(val: JsValue) -> Result<(), JsValue> {
    let example: Example = serde_wasm_bindgen::from_value(val)?;
    /* …do something with `example`… */

JavaScript Usage

In the JsValue that JavaScript gets, field1 will be a Map<number, string>, field2 will be an Array<Array<number>>, and field3 will be an Array<number>.

import { send_example_to_js, receive_example_from_js } from "example";

// Get the example object from wasm.
let example = send_example_to_js();

// Add another "Vec" element to the end of the "Vec<Vec<f32>>"
example.field2.push([5, 6]);

// Send the example object back to wasm.

Supported Types

Note that, even though it might often be the case, by default this library doesn't attempt to be strictly compatible with JSON, instead prioritising better compatibility with common JavaScript idioms and representations.

If you need JSON compatibility (e.g. you want to serialize HashMap<String, …> as plain objects instead of JavaScript Map instances), use the Serializer::json_compatible() preset.

By default, Rust ⬄ JavaScript conversions in serde-wasm-bindgen follow this table:

Rust JavaScript Also supported in from_value
() and Option<T>::None undefined null
bool boolean
f32, f64 number
u8, i8, …, u32, i32 number in the safe integer range
u64, i64, usize, isize number in the safe integer range bigint
u128, i128 bigint
String string
char single-codepoint string
Enum::Variant { … } as configured in Serde
HashMap<K, V>, BTreeMap, etc. Map<K, V> any iterable over [K, V]
Struct { key1: value1, … } { key1: value1, … } object
tuple, Vec<T>, HashSet, etc. T[] array any iterable over T
serde_bytes byte buffer Uint8Array ArrayBuffer, Array

The first two columns show idiomatic representations on Rust and JavaScript sides, while the 3rd column shows which JavaScript values are additionally supported when deserializing from JavaScript to the Rust type.

Serializer configuration options

You can customize serialization from Rust to JavaScript by setting the following options on the Serializer::new() instance (all default to false):

  • .serialize_missing_as_null(true): Serialize (), unit structs and Option::None to null instead of undefined.
  • .serialize_maps_as_objects(true): Serialize maps into plain JavaScript objects instead of ES2015 Maps.
  • .serialize_large_number_types_as_bigints(true): Serialize u64, i64, usize and isize to bigints instead of attempting to fit them into the safe integer number or failing.
  • .serialize_bytes_as_arrays(true): Serialize bytes into plain JavaScript arrays instead of ES2015 Uint8Arrays.

You can also use the Serializer::json_compatible() preset to create a JSON compatible serializer. It enables serialize_missing_as_null, serialize_maps_as_objects, and serialize_bytes_as_arrays under the hood.

Preserving JavaScript values

Sometimes you want to preserve original JavaScript value instead of converting it into a Rust type. This is particularly useful for types that can't be converted without losing the data, such as Date, RegExp or 3rd-party types.

serde_wasm_bindgen::preserve allows you to do just that:

#[derive(Serialize, Deserialize)]
pub struct Example {
    pub regular_field: i32,

    #[serde(with = "serde_wasm_bindgen::preserve")]
    pub preserved_date: js_sys::Date,

    #[serde(with = "serde_wasm_bindgen::preserve")]
    pub preserved_arbitrary_value: JsValue,

TypeScript support

There's no built-in type generation in this crate, but you can tsify with the js feature which integrates with serde-wasm-bindgen under the hood. Aside from generating structural typings, it also allows to derive IntoWasmAbi / FromWasmAbi so that you don't have to write from_value / to_value by hand.


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

Commit count: 188

cargo fmt