Theo's Typesafe Cult•16mo 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 Lind•16mo ago

prov is a list of many prisma row reads

Neto•16mo ago

that is the type of ProvScores?

Christian Lind•16mo ago

its a object from prisma or rather its a prisma type

Neto•16mo ago

let me test here

Christian Lind•16mo ago

thx!

Neto•16mo ago

just did this

Neto•16mo ago

Neto•16mo ago

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

Christian Lind•16mo ago

Christian Lind•16mo ago

hmm, i think it should be the correct type?

Neto•16mo ago

not there prov.reduce the type of prov

Christian Lind•16mo ago

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

Neto•16mo ago

hover over the prov variable

Christian Lind•16mo ago

yeah thats what i did?

Christian Lind•16mo ago

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

Neto•16mo ago

just weird overall did you restart the tsserver?

Christian Lind•16mo ago

yup

Christian Lind•16mo ago

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

Christian Lind•16mo ago

hovered over acc

Neto•16mo 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 Lind•16mo ago

Christian Lind•16mo ago

same error, does this code work on your machine?

Neto•16mo ago

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

Christian Lind•16mo ago

huh What was the difference between the projects?

Neto•16mo ago

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

Christian Lind•16mo ago

tried that also...

Christian Lind•16mo ago

same error here

stanisław•16mo 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 Lind•16mo ago

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

Neto•16mo ago

the ts server doing its inference

stanisław•16mo 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 Lind•16mo 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•16mo ago

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

Christian Lind•16mo 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•16mo 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 Lind•16mo ago

thanks for the help!

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Getting a possibly undefined after undefined check