Io-ts: Finding field names in a reporter

[spacejack]

Hi,

First, great library! Thanks for writing it, I'm having a lot of fun exploring it.

My biggest challenge so far is understanding the format of t.ValidationError. I'd like to build a reporter that returns a field errors dictionary object like: {[field: string]: string}.

With regular field I get one error with a context like ['', '', 'field'], but with unions, we get back 2 errors with a context like: ['', 'field', 0] and ['', 'field', 1].

I'm not sure how the context array is built. Would the first item ever be a non-empty string?

Is this a reliable way to retrieve the field name? i.e., by relying on the first non-empty key to be the field name?

function formatError (error: t.ValidationError) {
  const field = error.context.find(c => c.key.length > 0)
  return `Invalid ${field.key}`
}

BTW here is an example I'm playing with.

[spacejack]

Working with it a bit more, here's another example with form validation in case anyone else finds it useful or has suggestions.

[gcanti]

I'm not sure how the context array is built

function nullable<RT extends t.Any>(
  type: RT,
  name?: string
): t.UnionType<[RT, t.NullType], t.TypeOf<RT> | null, t.OutputOf<RT> | null, t.InputOf<RT> | null> {
  return t.union<[RT, t.NullType]>([type, t.null], name)
}

const rxEmail = /^[^@\s]+@[^@\s]+\.[^@\s]+$/
const Email = t.refinement(t.string, s => rxEmail.test(s), 'Email')

const User = t.interface(
  {
    id: t.Integer,
    username: t.string,
    email: nullable(Email, 'NullableEmail')
  },
  'User'
)

const result = User.decode({
  id: 123,
  username: 'fred',
  email: 'testtest.com' // '[email protected]'
})

if (result.isLeft()) {
  console.log(
    JSON.stringify(
      result.value.map(validationError =>
        validationError.context.map(contextEntry => `${contextEntry.key}: ${contextEntry.type.name}`)
      ),
      null,
      2
    )
  )
}
/*
[
  [
    ": User", // <= root, its key is an empty string
    "email: NullableEmail", // email field, key = "email"
    "0: Email" // first type of the NullableEmail union, key = 0
  ],
  [
    ": User", // root, the its is an empty string
    "email: NullableEmail", // email field, key = "email"
    "1: null" // second type of the NullableEmail union, key = 1
  ]
]
*/

TLDR

• Context = Array<ContextEntry> is an array representing the path to the offending value
• if the wrong value refers to a union type, you get an error for each member of the union
• the first ContextEntry usually refers to the root, hence its key is an empty string

[spacejack]

Hi, apologies for the additional noise, but again in case anyone finds it helpful...

I've been iterating on some patterns to show user-friendly errors from io-ts's error report. Here is one approach I'm working on: live demo and repo.