README.md

SwiftWasm Swift to JavaScript String Interoperation Example

Description

• Demonstrates passing string types from Swift to JS and vice versa.
• The primary files here are main.swift and main.mjs. The swift package is used to employ swift carton to build the wasm.

Getting started

Clone this repo and run the following command:

./demo.command

Expected output:

...
Hello From Swift!
[WASI stdout] Hello From JS!

How it works

Sending string from Swift to JS

Step 1 - call swift function from JS to populate string

main.mjs

// Call Swift function that populates the string to JS memory
const getStringFromSwiftFn = instance.exports.getStringFromSwift;
getStringFromSwiftFn() // ℹ️ String passed from Swift to JS here

Step 2 - Swift function populates string in js-readable memory

Sources/main.swift

/// Populates string to memory readable by JS.
@_expose(wasm, "getStringFromSwift")
@_cdecl("getStringFromSwift")
func getStringFromSwift() {
    JSObject.global.messageFromSwift = "Hello From Swift!".jsValue
}

Step 3 - JS parses and reads the memory value populated from Swift

main.mjs

// Get the message from Swift to JS
const messageFromSwift = globalThis.messageFromSwift
console.log(messageFromSwift)

Sending string from JS to Swift

Step 4 - Register receiving Swift closure from JS

main.mjs

const installSwiftMessengerClosureFn = instance.exports.installSwiftMessengerClosure;
installSwiftMessengerClosureFn() // Run this func to populate message in globalThis

Step 5 - Swift installation function sets up closure to receive the string and print out to console (as an example)

Sources/main.swift

/// Called by JS to install a closure that receives the string.
@_expose(wasm, "installSwiftMessengerClosure")
@_cdecl("installSwiftMessengerClosure")
func installSwiftMessengerClosure() {
    // 5
    let closure = JSClosure { jsValues in
        if let firstMessage = jsValues.compactMap({ $0.jsString }).first {
            print(firstMessage)
        }
        
        return .undefined
    }
    JSObject.global.swiftMessenger = closure.jsValue
}

Step 6 - Send the string from JS to Swift

main.mjs

// Send string to Swift
const swiftMessenger = globalThis.swiftMessenger
swiftMessenger("Hello From JS!") // ℹ️ String passed from JS to Swift here