Correct way to return error in custom service through GraphQL

Questions and Answers
3

Adapted to your needs it will look like this:

module.exports = {
  query: `
    createProduct(input: JSON): JSON
  `,
  resolver: {
    Mutation: { // Depends on your needs, you should use Query(for fetching data) or Mutation (for data modifications)
      createProduct: {
        description: 'Custom create product with validation',
        resolverOf: 'application::product.product.create', // Will apply the same policy on the custom resolver as the controller's action `find`.
        resolver: async (obj, options, ctx) => {
          let { product: requested_product, amount: requested_amount, client, deliverytime, type } = ctx.request.body;
          const product = await strapi.services.product.findOne({id: requested_product});

          //Will return an error custom message the amount differs
          if (requested_amount !== product.price) {
            return { status: '404', message: 'The order request is invalid.'};
          }
          //(other custom code)
        },
      },
    },
  },
};