Guides Responses

Handle validation errors

Use Pema schemas to validate inputs. When validation fails, Pema throws a ParseError. By default, Primate automatically turns this into a structured 400 Bad Request JSON response.

Default behavior

If you don't catch the error, Primate responds with a specific JSON format describing what failed.

This is the kind of behavior you want in most cases, as Primate's frontends have validation and form helpers to keep client values validated and synced with the server.

import pema from "pema";
import string from "pema/string";
import route from "primate/route";

const Signup = pema({
  email: string.email(),
});

route.post(request => {
  // throws a `ParseError` if the `email` field does not contain a valid email
  const { email } = request.body.fields(Signup);
  return `Signed up with ${email}`;
});

Custom JSON error

If you want custom behavior, wrap your validation in a try/catch block to override the default and return a custom response instead.

import ParseError from "pema/ParseError";

route.post(request => {
  try {
    const { email } = request.body.fields(Signup);
    return `Signed up with ${email}`;
  } catch (error) {
    if (error instanceof ParseError) {
      // returned as JSON
      return { error: "Please provide a valid email address." };
    }
    // will show an error page
    throw error;
  }
});

Custom error page

For frontend routes, you might prefer rendering a page rather than JSON.

primate/response bundles all standard response handlers such as json, text, and error.

import ParseError from "pema/ParseError";
import response from "primate/response";

route.post(request => {
  try {
    const { email } = request.body.fields(Signup);
    return `Signed up with ${email}`;
  } catch (error) {
    if (error instanceof ParseError) {
      return response.error("error.jsx", { message: "Invalid signup details" });
    }
    throw error;
  }
});