How to Use Shared Memory Between Rust WASM and JavaScript

Shared memory between Rust WASM and JavaScript is handled automatically via the WebAssembly linear memory buffer, which both environments access directly. You allocate memory in Rust using Vec or Box, export the data pointer and length to JavaScript, and read/write the buffer in JS using Uint8Array.

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub fn get_data_ptr() -> u32 {
    let data: Vec<u8> = vec![1, 2, 3, 4];
    let ptr = data.as_ptr() as u32;
    std::mem::forget(data); // Prevent deallocation so JS can read the memory
    ptr
}

#[wasm_bindgen]
pub fn get_data_len() -> u32 {
    4
}

import { get_data } from './pkg';
const ptr = get_data_ptr();
const len = get_data_len();
const memory = wasmInstance.exports.memory;
const buffer = new Uint8Array(memory.buffer, ptr, len);
console.log(buffer); // [1, 2, 3, 4]

Rust WASM and JavaScript share the same memory space, like two people looking at the same whiteboard. Rust writes data to specific spots on the board, and JavaScript reads those same spots directly without copying. This makes data transfer extremely fast because no translation or duplication is needed.