Validation

Validate form inputs with built-in and custom functions.

Built-in Validators

json-render includes common validation functions:

required — Value must be non-empty
email — Valid email format
minLength — Minimum string length
maxLength — Maximum string length
pattern — Match a regex pattern
min — Minimum numeric value
max — Maximum numeric value

Using Validation in JSON

Use { "$bindState": "/path" } on the value prop for two-way binding. Validation checks run against the value at the bound path (available as bindings?.value in components):

{
  "type": "TextField",
  "props": {
    "label": "Email",
    "value": { "$bindState": "/form/email" },
    "checks": [
      { "type": "required", "message": "Email is required" },
      { "type": "email", "message": "Invalid email format" }
    ],
    "validateOn": "blur"
  }
}

Validation with Parameters

{
  "type": "TextField",
  "props": {
    "label": "Password",
    "value": { "$bindState": "/form/password" },
    "checks": [
      { "type": "required", "message": "Password is required" },
      { 
        "type": "minLength", 
        "args": { "min": 8 },
        "message": "Password must be at least 8 characters"
      },
      {
        "type": "pattern",
        "args": { "pattern": "[A-Z]" },
        "message": "Must contain at least one uppercase letter"
      }
    ]
  }
}

Custom Validation Functions

Define custom validators in your catalog's functions field. The catalog itself is framework-agnostic — only the schema import varies by platform:

import { defineCatalog } from '@json-render/core';
import { schema } from '@json-render/react'; // or '@json-render/react-native'
import { z } from 'zod';

const catalog = defineCatalog(schema, {
  components: { /* ... */ },
  functions: {
    isValidPhone: {
      description: 'Validates phone number format',
    },
    isUniqueEmail: {
      description: 'Checks if email is not already registered',
    },
  },
});

Usage with React

In @json-render/react, use ValidationProvider to supply implementations for your custom validators:

import { ValidationProvider } from '@json-render/react';

function App() {
  const customValidators = {
    isValidPhone: (value) => {
      const phoneRegex = /^\+?[1-9]\d{1,14}$/;
      return phoneRegex.test(value);
    },
    isUniqueEmail: async (value) => {
      const response = await fetch(`/api/check-email?email=${value}`);
      const { available } = await response.json();
      return available;
    },
  };

  return (
    <ValidationProvider customFunctions={customValidators}>
      {/* Your UI */}
    </ValidationProvider>
  );
}

Using in Components

The useFieldValidation and useBoundProp hooks wire validation into your registry components. Validation uses the path from bindings?.value (the bound state path):

import { useFieldValidation, useBoundProp } from '@json-render/react';

function TextField({ props, bindings }) {
  const [value, setValue] = useBoundProp(props.value, bindings?.value);
  const { errors, isValid, validate, touch, clear } = useFieldValidation(
    bindings?.value ?? null,
    { checks: props.checks, validateOn: props.validateOn }
  );

  return (
    <div>
      <label>{props.label}</label>
      <input
        value={value || ''}
        onChange={(e) => setValue(e.target.value)}
        onBlur={() => validate()}
      />
      {errors.map((error, i) => (
        <p key={i} className="text-red-500 text-sm">{error}</p>
      ))}
    </div>
  );
}

See the @json-render/react API reference for full ValidationProvider and useFieldValidation documentation.

Validation Timing

Control when validation runs with validateOn:

change — Validate on every input change
blur — Validate when field loses focus
submit — Validate only on form submission

Learn about generation modes.