Schema Libraries (Yup, Zod)

Optional - delegate validation to Yup, Zod, or any schema library from inside a gform-react custom validator. gform's native, custom, and async rules already cover every case.

This is optional

gform-react validates everything on its own - native constraints, custom rules, and async checks cover every case without a schema library. Reach for Yup or Zod only if you already use one (e.g. a schema shared with your backend) and want a single source of truth.

Because custom validators are just functions, you can delegate to any schema library. Run the schema inside withCustomValidation and map its result to gform's true-means-error contract.

Zod

import { z } from "zod";
import { GValidator, type GValidators } from "gform-react";
 
const emailSchema = z.string().email();
 
const validators: GValidators<Form> = {
  email: new GValidator().withCustomValidation((input) => {
    const result = emailSchema.safeParse(input.value);
    if (!result.success) {
      input.errorText = result.error.issues[0].message;
      return true; // invalid
    }
    return false; // valid
  }),
};

Yup

Yup throws on failure, so map the catch to the same contract:

import { string } from "yup";
import { GValidator, type GValidators } from "gform-react";
 
const emailSchema = string().email("Enter a valid email").required("Required");
 
const validators: GValidators<Form> = {
  email: new GValidator().withCustomValidation((input) => {
    try {
      emailSchema.validateSync(input.value);
      return false; // valid
    } catch (err) {
      input.errorText = err.message;
      return true; // invalid
    }
  }),
};

Async schemas

For schema methods that return a promise (Zod's parseAsync, Yup's validate), use withCustomValidationAsync instead - the same true-means-error mapping applies to the resolved value.

The same pattern works with any validator you prefer - Valibot, Joi, a hand-written function - as long as you set input.errorText and return true on failure.

Custom Validation - the withCustomValidation contract in full, including cross-field rules.
GValidator - execution order and the mutation gotcha.

Zod

Yup

Next