Theo's Typesafe Cult•11mo ago

best practices to throw error on backend

I see people using throw new error and throw error But for me i kinda prefer having standard patterns across all the apis

return {
    status: false,
    error message 
}

return {
    status: false,
    error message 
}

17 Replies

Mocha•11mo ago

Errors in JavaScript world are too unpredictable and messy for throw to be useful to me. so yes, I'd either return

{ value: TValue | null, error: TError | null }

{ value: TValue | null, error: TError | null }

or if I don't care about the error message itself, I usually just

TValue | undefined

TValue | undefined

I'd say the first is a best practice for a library (that's what libraries like TanStack's do), and the other one is up to you to decide for your case 1

async function randomDog(breed: string): Promise<{url?: string, err?: string}> {
  return await fetch(`https://dog.ceo/api/breed/${breed}/images/random`)
    .then(res => res.json())
    .then(data => ({ url: data.message }))
    .catch(err => ({ err })) // could be user input error
}

const { err, url: huskyDog } = await randomDog('Husky')
if (err) return
console.log(huskyDog)

async function randomDog(breed: string): Promise<{url?: string, err?: string}> {
  return await fetch(`https://dog.ceo/api/breed/${breed}/images/random`)
    .then(res => res.json())
    .then(data => ({ url: data.message }))
    .catch(err => ({ err })) // could be user input error
}

const { err, url: huskyDog } = await randomDog('Husky')
if (err) return
console.log(huskyDog)

async function randomDogImage(): Promise<string | undefined> {
  return await fetch('https://dog.ceo/api/breeds/image/random')
    .then(res => res.json())
    .then(data => data.message)
    .catch(() => undefined) // non-user error
}

const dog = await randomDogImage()
if (!dog) return

async function randomDogImage(): Promise<string | undefined> {
  return await fetch('https://dog.ceo/api/breeds/image/random')
    .then(res => res.json())
    .then(data => data.message)
    .catch(() => undefined) // non-user error
}

const dog = await randomDogImage()
if (!dog) return

Benjamin•11mo ago

await + then() ? 🤔 Choose your weapon lol

Mocha•11mo ago

lol I only await the value I care about. I don't need res itself so I won't assign it to a variable

Benjamin•11mo ago

It looks weird to mix them up imo. The whole point of async await is to get rid of them() 😅 Plus it's confusing about what is actually returned here, you force the reader to pay more attention about what's finaly returned

Mocha•11mo ago

I agree it may be confusing, but especially with fetch and .json, I find it more appealing than:

async function awaits(): Promise<string | undefined> {
  const res = await fetch('https://dog.ceo/api/breeds/image/random')
  if (!res.ok) return undefined
  const data = await res.json()
  if (!data) return undefined
  return data.message
}
// or
async function saferAwaits(): Promise<string | undefined> {
  try {
    
    const res = await fetch('https://dog.ceo/api/breeds/image/random')
    const data = await res.json()
    return data.message
    
  } catch (err) {
    return undefined
  }
}

async function awaits(): Promise<string | undefined> {
  const res = await fetch('https://dog.ceo/api/breeds/image/random')
  if (!res.ok) return undefined
  const data = await res.json()
  if (!data) return undefined
  return data.message
}
// or
async function saferAwaits(): Promise<string | undefined> {
  try {
    
    const res = await fetch('https://dog.ceo/api/breeds/image/random')
    const data = await res.json()
    return data.message
    
  } catch (err) {
    return undefined
  }
}

I wouldn't use then in more complex cases, but for this one, I'll almost always do the same: fetch, json, & extract result. Do you have a cleaner way that also handles all possible errors?

Benjamin•11mo ago

The last one looks less confusing if I was the reviewer, I'd spend less time thinking about what's going on For me the last example you provided is good enough, it's clear

Mocha•11mo ago

Fair enough, and almost the same amount of code. I just have variable-phobia in JS I guess lol

Benjamin•11mo ago

Haha I understand, but at least it provides documentation without writing documentation 😁

scot22•11mo ago

I just use errors And throw them I worked on a project that used to use monads Then we stopped and ripped them out

Mocha•11mo ago

@Alex | C4 model stan How do you make sure all possible Errors are handled? Do you just always try-catch? If so, where do you try?

scot22•11mo ago

You can’t make sure all errors are handled, the program could always just crash I subscribe to the “let it crash” philosophy for some errors, and then for known errors I try to catch where it makes sense (as close as possible to the source)

barry•11mo ago

I never throw errors in utility functions. I return Error or Type Basically like Result in Rust Then I check where I use it If it's of type error, then there, I handle it. Instead of an ugly try catch block

const barrysWayIsTheWay = (): Promise<string | Error> => {
  const fetch_query = await fetch("").then((fetch_result) => fetch_result.json()).catch((error) => {
     return new Error("Fetching ...something... failed")
  })

  if (fetch_query instanceof Error) {
    return fetch_query
  }
  
  if (typeof fetch_query?.message !== "string") {
    return new Error("Message not of type string but of unexpected type " + typeof fetch_query?.message)
  }

  return fetch_query.message
}

const barrysWayIsTheWay = (): Promise<string | Error> => {
  const fetch_query = await fetch("").then((fetch_result) => fetch_result.json()).catch((error) => {
     return new Error("Fetching ...something... failed")
  })

  if (fetch_query instanceof Error) {
    return fetch_query
  }
  
  if (typeof fetch_query?.message !== "string") {
    return new Error("Message not of type string but of unexpected type " + typeof fetch_query?.message)
  }

  return fetch_query.message
}

I would validate it with Valibot too but you get the idea

Mocha•11mo ago

Love this, and yes, Rust's Result and Option are awesome.

export async function evenBetterer(): Promise<string | Error> {
    const fetch_query = await fetch('')
        .then(res => res.json() as Promise<{ message: string }>)
        .catch(() => new Error('Fetching ...something... failed'))

    if (typeof fetch_query?.message !== 'string')
        return new Error('Message not of type string')

    return fetch_query instanceof Error 
        ? fetch_query 
        : fetch_query.message
}

export async function evenBetterer(): Promise<string | Error> {
    const fetch_query = await fetch('')
        .then(res => res.json() as Promise<{ message: string }>)
        .catch(() => new Error('Fetching ...something... failed'))

    if (typeof fetch_query?.message !== 'string')
        return new Error('Message not of type string')

    return fetch_query instanceof Error 
        ? fetch_query 
        : fetch_query.message
}

barry•11mo ago

still need to check its of type string just because you cast it as it doesnt mean it is I tweaked the one above. Still need a ?. Accessing undefined will make the server shit itself

Mocha•11mo ago

That's a "this is fine" meme right here lol. With all of that, validation is still very much needed in serious apps I suspect Error would result in some weird behavior if used in tRPC since it's an instance of a class, not just a normal object? but otherwise I'd say this is clean and easy to work with "used in" as in, returned from a tRPC function and used like an Error class instance in client side

barry•11mo ago

Also, I highly recommend you look into https://ts-rest.com/

🪄 | ts-rest

Incrementally adoptable type-safety for your new and existing APIs

scot22•11mo ago

I agree with prime that using monads in ts is just too messy Great in rust Not so great in ts When I was a junior I used to use result types a lot but less so now

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best practices to throw error on backend