Theo's Typesafe Cult•2y ago

Getting a possibly undefined after undefined check

Getting really bugged with typescript here - what is going on here? The warning is that the object might possibly be undefined

36 Replies

Christian LindOP•2y ago

prov is a list of many prisma row reads

Neto•2y ago

that is the type of ProvScores?

Christian LindOP•2y ago

its a object from prisma or rather its a prisma type

Neto•2y ago

let me test here

Christian LindOP•2y ago

thx!

Neto•2y ago

just did this

Neto•2y ago

Neto•2y ago

and got no issue can you check prov the type on it

Christian LindOP•2y ago

Christian LindOP•2y ago

hmm, i think it should be the correct type?

Neto•2y ago

not there prov.reduce the type of prov

Christian LindOP•2y ago

Sorry where do you want me to check the type? where it is defined?

Neto•2y ago

hover over the prov variable

Christian LindOP•2y ago

yeah thats what i did?

Christian LindOP•2y ago

i just also sent you the definition of ProvScores which is the type of object in Prov 🙂

Neto•2y ago

just weird overall did you restart the tsserver?

Christian LindOP•2y ago

yup

Christian LindOP•2y ago

also tried to build my next project but get the same error

Christian LindOP•2y ago

hovered over acc

Neto•2y ago

just weird overall from tsserver paste this code where would be old ones just to be sure

const highestScore = prov.reduce((acc, cur) => {
  if (acc[cur.delprov]) {
    if (acc[cur.delprov].score < cur.score) {
      acc[cur.delprov] = cur;
    }
  } else {
    acc[cur.delprov] = cur
  }

  return acc
}, {} as Record<number, ProvScores>);

const highestScore = prov.reduce((acc, cur) => {
  if (acc[cur.delprov]) {
    if (acc[cur.delprov].score < cur.score) {
      acc[cur.delprov] = cur;
    }
  } else {
    acc[cur.delprov] = cur
  }

  return acc
}, {} as Record<number, ProvScores>);

Christian LindOP•2y ago

Christian LindOP•2y ago

same error, does this code work on your machine?

Neto•2y ago

i got no issue on bun let me try another project now i got that issue

Christian LindOP•2y ago

huh What was the difference between the projects?

Neto•2y ago

im checking that rn is some config on tsconfig honestly just do acc[cur.devprov]?.score < cur.score with the ?

Christian LindOP•2y ago

tried that also...

Christian LindOP•2y ago

same error here

stanisław•2y ago

guys the error is correct To fix it you'd have to make something like this

const currValue = acc[cur.delprov]

if(currValue){
  if(currValue.score < cur.score)
}

const currValue = acc[cur.delprov]

if(currValue){
  if(currValue.score < cur.score)
}

if u assign it to a variable then ts will know its not undefiend after the check

Christian LindOP•2y ago

okay so that removed my error - but why would the method that I and @Neto were investigating be incorrect

Neto•2y ago

the ts server doing its inference

stanisław•2y ago

becuase after you check an object property like this acc[cur.delprov] ts has no insurance that you are not mutating an object afterwards once you assign it then ts knows "currValue is x"

Christian LindOP•2y ago

ah okay, so if i would have instead said let currValue = ..., then that would also give an error since then the compiler can't know if the variable has changed value

stanisław•2y ago

No, you wouldn't get an error if the type was correct

Christian LindOP•2y ago

but if it is a mutable variable i could reassign it to be undefined, no? Or is that when the type error would kick in

Neto•2y ago

the let only implies that you can change the value using let currVal = null then you have some issues with inferring the type

Christian LindOP•2y ago

thanks for the help!

Gaming

Programming

Getting a possibly undefined after undefined check