custom-errors.js

import { assert, number, object, string } from 'superstruct'

// Define a struct to validate with.
const User = object({
  id: number(),
  name: string(),
  email: string(),
})

// Define data to be validated.
const data = {
  id: 1,
  name: true,
  email: 'jane@example.com',
}

// Validate the data. In this case the `name` property is invalid, so an error
// will be thrown that you can catch and customize to your needs.
try {
  assert(data, User)
} catch (e) {
  const { key, value, type } = e

  if (value === undefined) {
    const error = new Error(`user_${key}_required`)
    error.attribute = key
    throw error
  }

  if (type === 'never') {
    const error = new Error(`user_attribute_unknown`)
    error.attribute = key
    throw error
  }

  const error = new Error(`user_${key}_invalid`)
  error.attribute = key
  error.value = value
  throw error
}

// Error: 'user_name_invalid' {
//   attribute: 'name',
//   value: true,
// }