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GraphQL TOE

Like bumping your toe on something... I usually throw things around after that happens
-- Pascal Senn, ChilliCream

TOE: Throw On Error

What is it?

Takes a GraphQL response with data and errors and returns a value equivalent to the data value, except if you read from a field that errored, the error will be thrown (rather than being null and you having to look up the error in errors yourself manually).

This allows you to handle GraphQL errors more naturally in your client code, e.g. with try/catch or via <ErrorBoundary />, and also means you can rely on the "semantic nullability" of your GraphQL schema, rather than the "strict nullability" (i.e. you need to do fewer null checks since more of your types can be non-nullable, even if errors might occur there).

See "more details" below for... well... more details.

Zero-dependencies

This is a simple single-js-file module, it shouldn't add much to your bundle size (v0.1.1 requires just 468 bytes gzipped according to bundlephobia) and can be used with any client that will provide you data and errors - even fetch()!

If you want the errors thrown to be of a particular class (e.g. GraphQLError or just Error) then you should map them before feeding to toe() - we just throw the raw error object you pass in.

Installation

Pick the line that relates to your package manager:

npm install --save graphql-toe
yarn add graphql-toe
pnpm install --save graphql-toe

Usage

Overview

import { toe } from "graphql-toe";

// const result = do something returning an object with { data, errors }
const data = toe(result);

data now represents the combination of result.data and result.errors, such that it's identical to result.data in the case that no errors occur, and otherwise it's recursively modified to replace errored fields with a getter that throws. In the case that data itself is null or undefined, toe(result) itself will throw an error.

e.g.

import { toe } from "graphql-toe";

// Example data from GraphQL
const result = {
  data: {
    deep: {
      withList: [
        { int: 1 },
        {
          /* `null` because an error occurred */
          int: null,
        },
        { int: 3 },
      ],
    },
  },
  errors: [
    {
      message: "Two!",
      // When you read from this path, an error will be thrown
      path: ["deep", "withList", 1, "int"],
    },
  ],
};

// TOE'd data:
const data = toe(result);

// Returns `3`:
data.deep.withList[2].int;

// Returns an object with the key `int`
data.deep.withList[1];

// Throws the error `Two!`
data.deep.withList[1].int;

How to get result and feed it to toe(result) will depend on the client you're using. Here are some examples:

URQL

An exchange for "throw on error" has been submitted here: urql-graphql/urql#3677

Apollo Client

Get data and error from useQuery(); then extract errors from error?.graphQLErrors. Combine these into a response object (const response = { data, errors }) and feed to toe(). The result is your TOE'd data. Of course, this is no good if Apollo never returns you partial data, so you must also use errorPolicy to return 'all' errors.

For example, you might use a replacement to useQuery() such as:

import { toe } from "graphql-toe";

function useQueryTOE(document, options) {
  // TODO: test me!
  const { data: rawData, error } = useQuery(document, { errorPolicy: 'all', ...options });
  const result = { data: rawData, errors: error?.graphQLErrors }
  const data = toe(result);
  return data;
}

Note similar changes should be made to mutation and subscription operations.

fetch()-based clients

GraphQL clients that return the server response directly (e.g. fetch(), graffle, etc) can just feed the result directly into toe(result):

import { toe } from "graphql-toe";

// const response = await fetch('/graphql', { method: "POST", headers: ..., body: ... });
// if (!response.ok) throw new Error("Uh-oh!");

const result = await response.json()
const data = toe(result);

More details

On the server side, GraphQL captures errors, replaces them in the returned data with a null, and adds them to the errors object. Clients typically then have to look at data and errors in combination to determine if a null is a "true null" (just a null value) or an "error null" (a null with a matching error in the errors list). This is unwieldy.

This project re-introduces thrown errors into your data, by walking the data tree and replacing any errored fields with "getters" which means when you read a field that has errored, you have an error thrown in your client! This can therefore leverage JavaScript's natural error handling semantics (i.e. try/catch), and also means that it can integrate naturally with things like React's <ErrorBoundary /> component!

TODO

  • Add support for incremental delivery
  • Add an optimized toe() where all inputs are assumed to be null-prototype objects, making key traversal faster
  • Add an optimized toe() where inputs are modified in-place (mutated) for maximum performance

History

Version 0.1.0 of this module was released from the San Francisco Centre the day after GraphQLConf, following many fruitful discussions around nullability.