SwiftWasm Blog

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SwiftWasm 5.5.0 is now available

We're happy to announce the new release of SwiftWasm tracking upstream Swift 5.5! Notably, in this release we've added support for async/await. This new feature of Swift can be integrated with JavaScript promises when you're using a new version of JavaScriptKit that was recently tagged. See the corresponding section below for more details.

Since multi-threading in WebAssembly is still not supported across all browsers (Safari is the only one lagging behind), this release of SwiftWasm doesn't include the Dispatch library and ships with a single-threaded cooperative executor. This means that actor declarations in your code will behave as plain reference types and will all be scheduled on the main thread. If you need true parallel computation, you’ll have to write custom code against the Web Workers API (either via JavaScriptKit or delegating to raw JavaScript) to synchronize multiple SwiftWasm runtimes.

Additionally, 5.5.0 is the first release of SwiftWasm that supports Apple Silicon natively. The latest version of carton (0.12.0) will download the arm64 distribution on Apple Silicon devices.

New JavaScriptKit runtime

The 0.11 release of JavaScriptKit adds support for async/await and JSPromise integration. Now instances of this class have an effectful async property value. Here's example code that shows you how can fetch browser API be used without callbacks:

import JavaScriptKit
import JavaScriptEventLoop

// This line is required for `JSPromise.value` to work.

private let jsFetch = JSObject.global.fetch.function!
func fetch(_ url: String) -> JSPromise {

struct Response: Decodable {
    let uuid: String

let alert = JSObject.global.alert.function!
let document = JSObject.global.document

var asyncButtonElement = document.createElement("button")
asyncButtonElement.innerText = "Fetch UUID demo"
asyncButtonElement.onclick = .object(JSClosure { _ in
    Task {
        do {
            let response = try await fetch("https://httpbin.org/uuid").value
            let json = try await JSPromise(response.json().object!)!.value
            let parsedResponse = try JSValueDecoder().decode(Response.self, from: json)
        } catch {

    return .undefined

_ = document.body.appendChild(asyncButtonElement)

Also, in this version of JavaScriptKit we're simplifying the JSClosure API. You no longer need to release instances of this class manually, as they will be automatically garbage-collected by the browser after neither your Swift code nor the JavaScript runtime hold any references to them. This is achieved with the new

FinalizationRegistry Web API, for which we had to significantly increase minimum browser versions required for JavaScriptKit to work. See README.md in the project repository for more details.

We have to mention that there's still a possibility of reference cycles with this new API. FinalizationRegistry doesn't implement full GC for JS closures, but it only solves dangling closure issue. For example, in this code

var button = document.createElement("button")
button.onclick = .object(JSClosure { [button] in
  // this capture makes a reference cycle

a reference cycle is created

┌─> JSObject (button in Swift) -> HTMLElement (button in JS) ────┐
└── JSClosure (onclick in Swift) <─> Closure (onclick in JS) <───┘

In this case, when button element is removed from the main DOM tree, it cannot be deallocated. The onclick closure is still referenced by the button itself. These reference cycles can be resolved with the usual weak captures you're probably used to writing in your AppKit and UIKit code.


Based on the improvements to JavaScriptKit and major work by our contributors, we're also tagging a new 0.9.0 release of Tokamak, a SwiftUI-compatible framework for building browser apps with WebAssembly. We've added:

  • Canvas and TimelineView types;
  • onHover modifier;
  • task modifier for running async functions;
  • Sanitizers for the Text view.

Tokamak v0.9.0 now requires Swift 5.4 or newer. Swift 5.5 (with SwiftWasm 5.5 when targeting the browser environment) is recommended.


We'd like to thank our sponsors for their support, which allowed us to continue working on the SwiftWasm toolchain and related projects.

Many thanks to MacStadium for giving us access to Apple Silicon hardware. Without their help it would be close to impossible to set up CI for enabling full M1 support in our toolchain.

Additionally, we'd like to thank everyone who contributed their work and helped us make this release happen. These new releases wouldn't be possible without the hard work of (in alphabetical order):