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Validating user input is usually required both on the client-side and on the server-side, often causing a violation of the DRY design principle. With Remult, validation code can be placed within the entity class, and Remult will run the validation logic on both the frontend and the relevant API requests.

Handling validation errors

When a validation error occurs, Remult will throw an exception.

In this tutorial, we will catch these exceptions, and alert the user.

Validate the Title Field

Task titles are required. Let's add a validity check for this rule.

  1. In the Task entity class, modify the Fields.string decorator for the title field to include an object literal argument and set the object's validate property to Validators.required.
import { Validators } from 'remult';

  validate: Validators.required
title: string = '';
  1. In +page.svelte, sorround the addTask in a try-catch block to capture the error:
let newTaskTitle = ''
const addTask = async () => {
  try { 
    const newTask = await remult.repo(Task).insert({ title: newTaskTitle })
    tasks = [...tasks, newTask]
    newTaskTitle = ''
  } catch (error) { 
    alert((error as { message: string }).message) 

After the browser is refreshed, try creating a new task or saving an existing one with an empty title - the "Title: Should not be empty" error message is displayed.

Sorround all the other functions in try-catch in a similar manner and notify the user accordingly.

Implicit server-side validation

The validation code we've added is called by Remult on the server-side to validate any API calls attempting to modify the title field.

Try making the following POST http request to the http://localhost:5173/api/tasks endpoint, providing an invalid title.

curl -i http://localhost:5173/api/tasks -d "{\"title\": \"\"}" -H "Content-Type: application/json"

A HTTP 400 Bad Request error is returned and the validation error text is included in the body:

  modelState: { title: 'Should not be empty' },
  message: 'Title: Should not be empty'


You should probably update all your code to handle these errors gracefully with try-catch blocks.

Custom Validation

Remult accords you the ability to easly create your own validation rules.

The validate property allows an arrow function which accepts an instance of the entity to be validated. This function will be called to validate input on both front-end and back-end.

Try something like this and see what happens:

  validate: (task) => {
    if (task.title.length < 3) throw "The title must be at least 3 characters long"
title = ""

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