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Clavin June
Clavin June

Posted on • Originally published at clavinjune.dev on

Golang WASM Async Function

Sunday Snippet #8 golang WASM async function

package main

import (
    "fmt"
    "syscall/js"
)

type fn func(this js.Value, args []js.Value) (any, error)

var (
    jsErr     js.Value = js.Global().Get("Error")
    jsPromise js.Value = js.Global().Get("Promise")
)

func asyncFunc(innerFunc fn) js.Func {
    return js.FuncOf(func(this js.Value, args []js.Value) any {
        handler := js.FuncOf(func(_ js.Value, promFn []js.Value) any {
            resolve, reject := promFn[0], promFn[1]

            go func() {
                defer func() {
                    if r := recover(); r != nil {
                        reject.Invoke(jsErr.New(fmt.Sprint("panic:", r)))
                    }
                }()

                res, err := innerFunc(this, args)
                if err != nil {
                    reject.Invoke(jsErr.New(err.Error()))
                } else {
                    resolve.Invoke(res)
                }
            }()

            return nil
        })

        return jsPromise.New(handler)
    })
}

func argsEvenOdd(this js.Value, args []js.Value) (any, error) {
    if len(args)%2 == 0 {
        return nil, fmt.Errorf("#args is even")
    }
    return "#args is odd", nil
}

func main() {
    js.Global().Set("argsEvenOdd", asyncFunc(argsEvenOdd))
    select {}
}
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Test it with

argsEvenOdd(1, 2)
        .then(res => console.log(res)) 
        .catch(err => console.error(err)) // will be catched

argsEvenOdd(1)
        .then(res => console.log(res)) // will be logged
        .catch(err => console.error(err))
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