To modify other form fields based on changes in specific fields using React Hook Form’s watch and setValue functions, you can utilize the watch function to monitor changes in form fields and update other fields accordingly using setValue. Here’s how you can achieve this:

Step 1: Install React Hook Form

If you haven’t already installed React Hook Form, you can do so using npm or yarn:

npm install react-hook-form

or

yarn add react-hook-form

Step 2: Set Up Your Form with React Hook Form

Create a form component and set up your form fields using React Hook Form.

import React from 'react';
import { useForm } from 'react-hook-form';

const FormExample = () => {
  const { register, watch, setValue } = useForm();
  const watchField = watch('fieldName');

  // Function to update other fields based on changes in 'fieldName'
  const updateOtherFields = () => {
    if (watchField === 'someValue') {
      setValue('otherField', 'newValue');
    } else {
      setValue('otherField', 'defaultValue');
    }
  };

  // Watch 'fieldName' for changes and update other fields accordingly
  React.useEffect(() => {
    updateOtherFields();
  }, [watchField]);

  return (
    <form>
      <input type="text" {...register('fieldName')} />
      <input type="text" {...register('otherField')} />
    </form>
  );
};

export default FormExample;

In this example, we have two form fields: 'fieldName' and 'otherField'. We’re using watch to monitor changes in 'fieldName' and then calling the updateOtherFields function to update 'otherField' based on the value of 'fieldName'.

Step 3: Run Your Form Component

Include your FormExample component in your application:

import React from 'react';
import ReactDOM from 'react-dom';
import FormExample from './FormExample';

const App = () => {
  return (
    <div>
      <h1>React Hook Form Example</h1>
      <FormExample />
    </div>
  );
};

ReactDOM.render(<App />, document.getElementById('root'));

Summary

By using watch and setValue from React Hook Form, you can easily monitor changes in specific form fields and update other fields accordingly. This approach provides a simple and efficient way to manage form state and dependencies in React applications.