Predicate().error()

Adds state callbacks which will be invoked when an error occurs in predicate functions or in their state callbacks during validation process.

Syntax

Predicate().error(callback1, callback2, /* ..., */ callbackN)

Parameters

callback1, ..., callbackN
Functions to add. The callback functions will be called with the following arguments:
- error
  An Error object.
- next
  A function to invoke in case the error is not needed to be catched and instead is needed to be passed further.

Return value

The Predicate object.

Exceptions

If anything other than a function is passed in the corresponding error will be thrown.

Description

Adds error state callbacks to the Predicate object. Once the Predicate is in the error state, the callbacks will be invoked one by one in the order they were added. The main purpose of error state callbacks is to perform side effects related to the error state.

This method catches errors that occur in predicate functions and in their associated state callbacks when the Validation.validate() method gets invoked (or the Validation object is used as an event handler or middleware).

If you want to just perform side effects and not to catch the error on this level, it is possible to forward the error further to catch it on the Validation().error() level by calling the function passed into a callback as the parameter next. If you add several error state callbacks in a row, calling the function next in only one of them is enough.

Also see “Error handling”.

Examples

Catching an error in a predicate function

import { Validation, Predicate } from "isomorphic-validation";

// predicate function
const faultyCheckEmail = (value) =>
    fetch(`http://your-app.com/check-email/?q=${value}`)
        .then(res => res.json());

const errorHandler = ({message}) => { console.log(message); };

const validatableObject = { value: 'a@a.a' };

const validation = Validation(validatableObject)
    .constraint(
        Predicate(faultyCheckEmail)
            .error(errorHandler)
    );

await validation.validate();
await validation.validate();

// Output:
//
// fetch failed
// fetch failed

Catching an error in a state callback of a `Predicate`

import { Validation, Predicate } from "isomorphic-validation";

// predicate function
const isMeaningOfLife = (value) => value === 42;

// state callbacks
const faultyCallback = (res) => { res.a.b.c }; // accessing a non existing property
const errorHandler = ({message}) => { console.log(message); };

const validatableObject = { value: 42 };

const validation = Validation(validatableObject)
    .constraint(
        Predicate(isMeaningOfLife)
            .validated(faultyCallback)
            .error(errorHandler)
    );

await validation.validate();
await validation.validate();

// Output:
//
// Cannot read properties of undefined (reading 'b')
// Cannot read properties of undefined (reading 'b')

Error forwarding

import { Validation, Predicate } from "isomorphic-validation";

const { log } = console;

// predicate functions

const faultyCheckEmail = (value) =>
    fetch(`http://your-app.com/check-email/?q=${value}`)
        .then(res => res.json());

const areAllowedChars = (value) => /^[A-Za-z\s]+$/.test(value);

// state callbacks

const emailValidHandler = () => { log('The e-mail is available.'); };
const emailInvalidHandler = () => { log('The e-mail is taken.'); };
const emailErrorHandler = ({message}) => { log('e-mail:', message, '\n'); };

const nameValidHandler = () => { log('The name has valid characters.'); };
const nameInvalidHandler = () => { log('The name has invalid characters.'); };
const nameErrorHandler = ({message}) => { log('name:', message); };

const predicateErrorHandler = (err, next) => { log('Predicate:', err.message); next() };
const groupErrorHandler = (err, next) => { log('group:', err.message); next() };
const promiseRejectHandler = ({ message }) => { log('Promise:', message, '\n'); };

// validatable objects
const email = { value: 'a@a.a' };
const name = { value: 'John Doe' };

const emailValidation = Validation(email)
    .constraint(
        Predicate(faultyCheckEmail)
            .error(predicateErrorHandler)
    )
    .valid(emailValidHandler)
    .invalid(emailInvalidHandler)
    .error(emailErrorHandler);

const nameValidation = Validation(name)
    .constraint(areAllowedChars)
    .valid(nameValidHandler)
    .invalid(nameInvalidHandler)
    .error(nameErrorHandler);

const validation = Validation.group(emailValidation, nameValidation)
    .error(groupErrorHandler);

// catching the forwarded error
await validation.validate().catch(promiseRejectHandler); // predicateErrorHandler, groupErrorHandler and promiseRejectHandler will be called

await emailValidation.validate(); // predicateErrorHandler and emailErrorHandler will be called
await nameValidation.validate(); // nameValidHandler will be called (no errors)

// Output:
//
// Predicate: fetch failed
// group: fetch failed
// Promise: fetch failed
//
// Predicate: fetch failed
// e-mail: fetch failed
//
// The name has valid characters.