asked on

nodejs async code with functions that use callbacks and do not return promises

i am struggling to understand nodejs asynchronous code better

consider the function fs.realpath() which does not return a promise and expects a callback

Q1 : is there a variant of the func that i can call simply with

realpath = async realpath(whatever)

Q2 : is there a generic way to allow async code to run in the callback. i assume with promisify

await promisify(fs.realpath(whatever,(err,function(){

// do something using async here

})))

Q3 : is there some kind of trivial hack that will allow to set a variable in the parent scope within the callback while preserving async scopes properly. i do not really care if the call is blocking but if it can be asynchronous all the better.

var path

fs.realpath(whatever,(err,function(){
// set the path here
}))

// do something using path here

i must admit i read the documentation but the learning curve is rather huge. i am rather familiar with all kinds of asynchronous programing including event driven but the nodejs way is still quite blurry

thanks a lot for your help

skullnobrains

ASKER

got part of my own answer but still eager to get some input

this works for Q2
i would be happy to know if there is anything simpler or something i might be missing that would mess the scopes if i use this syntax

const realpath=util.promisify(myfunc)
await myfunc("PARAMETER", async function (err,url){
// async code here
})

David Favor

Q1: Impossible, as what you're describing is synchronous (call, work, wait for return) type coding.

You can simulate this with some odd code + net result is the same, you're forcing async code to run synchronously, so just nullified the async function.

skullnobrains

ASKER

Q1

the goal is to get a return value rather than a callback
this is actually handled quite nicely by promisify
promisifying the func actually allows to omit the callback
"async" was a typo for "await" my bad

rp = await realpath(p).catch((error) => { console.log(error) })
if(rp === undefined)return
console.log(rp)

Open in new window

try {
    rp = await realpath(p)
    console.log(rp)
}catch(e){console.log(e)}

Open in new window

the catch block may be omitted as long as the whole thing is itself in some catch block
i see no way to handle the error outside of a catch block

anyone who is interested may dig deeper using new promise and resolve but my attempts merely produced at best the same results with a much more complex syntax

---

Q3 has an easy answer in global scope
but this won't help here since the callbacks are ALWAYS run asynchronously which i was totally missing

there won'y be a way to hack something into the parent scope since it simply does not exist when the callback is fired unless await was used. in that case, promisify does the job so there is no point bothering.

skullnobrains

ASKER

@david : this is the whole point of async/await (no need to maintain scopes manually and no unreasonable number of nested stuff) : they allow to code the old style way while still running asynchronous. and they take care of the scope adequately. at least that is what i am hoping for. and still testing

ASKER CERTIFIED SOLUTION

skullnobrains

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