JavaScript Platform

Buffer on JS wraps Uint8Array with optional SharedArrayBuffer support.

Implementation

Zone	JS Type
Heap	Uint8Array (ArrayBuffer)
Direct	Uint8Array (ArrayBuffer)
SharedMemory	Uint8Array (SharedArrayBuffer)

SharedArrayBuffer

For multi-threaded scenarios (Web Workers):

val buffer = PlatformBuffer.allocate(1024, AllocationZone.SharedMemory)

// Access the SharedArrayBuffer
val jsBuffer = buffer as JsBuffer
// jsBuffer.sharedArrayBuffer  // Only set if SharedMemory zone

CORS Requirements

SharedArrayBuffer requires specific HTTP headers:

Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp

Webpack Configuration

Add to webpack.config.d/headers.js:

if (config.devServer != null) {
    config.devServer.headers = {
        "Cross-Origin-Opener-Policy": "same-origin",
        "Cross-Origin-Embedder-Policy": "require-corp"
    }
}

Without these headers, falls back to regular ArrayBuffer.

Node.js vs Browser

Node.js

// Works with Node.js Buffer
val buffer = PlatformBuffer.allocate(1024)
buffer.writeString("Hello from Kotlin/JS!")

Browser

// Works with browser APIs
val buffer = PlatformBuffer.allocate(1024)

// Can be used with Fetch API, WebSocket, etc.
buffer.writeBytes(responseData)

Web Worker Communication

// Main thread
val sharedBuffer = PlatformBuffer.allocate(1024, AllocationZone.SharedMemory)
sharedBuffer.writeInt(42)

// Pass to worker (zero-copy with SharedArrayBuffer)
worker.postMessage(sharedBuffer.asArrayBuffer())

// Worker thread
val buffer = PlatformBuffer.wrap(receivedArrayBuffer)
val value = buffer.readInt()  // 42

Native Data Conversion

Convert buffers to JavaScript-native types for Web API interop:

val buffer = PlatformBuffer.allocate(1024)
buffer.writeBytes(data)
buffer.resetForRead()

// Get ArrayBuffer (for Fetch, WebSocket, etc.)
val nativeData = buffer.toNativeData()
val arrayBuffer: ArrayBuffer = nativeData.arrayBuffer
fetch(url, RequestInit(body = arrayBuffer))

// Get Int8Array (mutable view)
val mutableData = buffer.toMutableNativeData()
val int8Array: Int8Array = mutableData.int8Array

Zero-Copy Behavior

Conversion	JsBuffer
toNativeData()	Zero-copy for mutable buffers, copy for read-only
toMutableNativeData()	Zero-copy (subarray view)
toByteArray()	Zero-copy (subarray)

ArrayBuffer Limitation

ArrayBuffer itself is immutable (you can't create a view of a portion). When converting to ArrayBuffer, a copy may be needed for partial views. Int8Array supports zero-copy subarray views.

Web API Examples

// Fetch API
val response = fetch(url).await()
val arrayBuffer = response.arrayBuffer().await()
val buffer = PlatformBuffer.wrap(arrayBuffer)

// WebSocket
webSocket.onmessage = { event ->
    val data = event.data as ArrayBuffer
    val buffer = PlatformBuffer.wrap(data)
    processMessage(buffer)
}

// Send via WebSocket
val buffer = PlatformBuffer.allocate(1024)
buffer.writeInt(messageId)
buffer.writeString(payload)
buffer.resetForRead()
webSocket.send(buffer.toNativeData().arrayBuffer)

See Platform Interop for more details.

Best Practices

1. Configure CORS headers for SharedArrayBuffer
2. Use SharedMemory for Worker communication
3. Pool buffers for WebSocket message handling
4. Handle fallback when SharedArrayBuffer unavailable