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GraphQL Codegen for Fast Check

What?

GraphQL Code Generator lets you generate code from a GraphQL schema. It is commonly used to create Typescript types to ensure the consuming program agrees with the data it is receiving.

Fast-check is a Property Testing framework for Typescript / Javascript. It allows testing properties of a program using arbitrarily generated data. Fast-check uses arbitrarys to create data, that is, descriptions of the data we need to create.

Say we have this datatype...

// our datatype
interface Person {
  name: string;
  age: number;
}

...the code to make the fast-check arbitrary would look like:

import * as fc from "fast-check";

const personArbitrary = fc.record({
  name: fc.string(),
  age: fc.integer()
});

This can then be used in property tests or as generators to create sample test data. The problem is that writing them all by hand and keeping them up to date with your datatypes is a pain.

This library

This library is a plugin for GraphQL Code Generator that creates fast-check Arbitrarys for your schema data types.

How to use

Add the library to your project (and GraphQL Codegen if you need it):

yarn add graphql-codegen-fast-check @graphql-codegen/cli

Create a codegen.yml file.

schema: path-to-my-graphql-schema.graphql
generates:
  output.ts:
    - graphql-codegen-fast-check

Then run the generator in the same folder as your codegen.yml file:

yarn graphql-codegen

(hopefully) Success! You should have an output.ts folder full of arbitrarys you can use in your project.

What's working?

• Object types
• Built-in Scalar types
• Enumeration types
• Type modifiers (1)
• Custom Scalar types (2)
• Union types
• Interface types (3)
• Input types (4)
• Documents (5)

1. Currently lists work but we ignore the nullable / non-nullable distinction. Everything is non-nullable for now, this will come soon.
2. Custom Scalar types are defined outside the schema, frustratingly. For now they all emit a string, the plan is to allow the config to specify a file to override these with the user's own arbitrary instances.
3. These "work" but need to check whether they are actually correct
4. Currently these just output a string (why? don't know) - fast-check has a func arbitrary which should make this fairly straightforward to implement.
5. I haven't even thought about tackling this part yet.

Other notes

Recursive declarations will utterly explode if you try and use them. I have a plan to implement a limit to stop this.

Inspiration

Thanks to fast-check-io-ts for showing me that making arbitrary instances should be easy.