How to update a field in a document from the Gatsby frontend using Sanity.io

export default {
  name: 'problem',
  type: 'document',
  title: 'Problems',
  fields: [
    {
      name: 'title',
      title: 'Title',
      type: 'string',
      validation: (Rule) => Rule.required(),
    },
    {
      name: 'rating',
      title: 'Number of votes',
      type: 'number',
      validation: (Rule) =>
        Rule.required().integer().positive().error('Must be a positive integer!'),
    },
  ],
};

const sanityClient = require('@sanity/client');

const client = sanityClient({
  projectId: process.env.GATSBY_SANITY_PROJECT_ID,
  dataset: 'production',
  token: process.env.SANITY_TOKEN_WRITE,
  useCdn: false,
});

exports.handler = async function (event, context) {
  const body = JSON.parse(event.body);
  const { id } = body;

  const result = await client
    .patch(id) // Document ID to patch
    .inc({ rating: 1 }) // Increment field by count
    .commit() // Perform the patch and return a promise
    .then((updatedRating) => {
      console.log('Merci pour le vote!');
      console.log(updatedRating);
    })
    .catch((err) => {
      console.error('Désolé, il y a eu un souci : ', err.message);
    });

  return {
    statusCode: 200,
    body: JSON.stringify({
      message: `Merci pour votre vote !`,
    }),
  };
};