Generating Fake Data From JSON Schema

This post will demonstrate the usage of json-schema-faker to generate fake data from a JSON schema document.

Prerequisites

Getting started

Ensure that you have this in the file __generated__/schema.json:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "definitions": {
    "User": {
      "type": "object",
      "properties": {
        "id": {
          "type": "string"
        },
        "email": {
          "type": ["string", "null"]
        },
        "name": {
          "type": ["string", "null"],
          "x-faker": "name.findName"
        },
        "emailVerified": {
          "anyOf": [
            {
              "type": "string",
              "format": "date-time"
            },
            {
              "type": "null"
            }
          ]
        },
        "image": {
          "type": ["string", "null"]
        }
      },
      "required": ["id", "email", "name", "emailVerified", "image", "role"],
      "additionalProperties": false,
      "description": "Model User"
    },
    "Post": {
      "type": "object",
      "properties": {
        "id": {
          "type": "string"
        },
        "createdAt": {
          "type": "string",
          "format": "date-time"
        },
        "updatedAt": {
          "type": "string",
          "format": "date-time"
        },
        "title": {
          "type": "string"
        },
        "content": {
          "type": ["string", "null"]
        },
        "published": {
          "type": "boolean"
        },
        "authorId": {
          "type": "string"
        }
      },
      "required": [
        "id",
        "createdAt",
        "updatedAt",
        "title",
        "content",
        "published",
        "authorId"
      ],
      "additionalProperties": false,
      "description": "Model Post"
    }
  }
}
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Assuming you already have the npm project initialized you will also need to add a few packages:

$ npm i json-schema-faker faker cuid
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Writing our script

With that there, we can create the file scripts/generate-data-from-json-schema.js and add the following:

const jsf = require("json-schema-faker");
const fs = require("fs");
const path = require("path");

const schema = JSON.parse(
  fs.readFileSync(path.join(__dirname, "../__generated__/schema.json"), "utf8")
);
const dataPath = path.join(__dirname, "../__generated__/data.json");

// Extend the base functionality
jsf.extend("faker", () => require("faker"));
jsf.extend("cuid", () => {
  const cuid = require("cuid");

  const res = {
    cuid: () => cuid(),
  };
  return res;
});

const data = {};

async function main() {
  let currentData;
  if (fs.existsSync(dataPath)) {
    currentData = JSON.parse(fs.readFileSync(dataPath, "utf8"));
  }

  const entries = Object.entries(schema.definitions);

  for (const [key, value] of entries) {
    if (currentData && currentData[key]) {
      console.log("Using existing data for", key);
      data[key] = currentData[key];
      continue;
    }

    const injected = {
      ...value,
      definitions: schema.definitions,
    };

    // use the async-version (preferred way)
    const sample = await jsf.resolve(injected);
    data[key] = sample;
  }

  const outputJson = JSON.stringify(data, null, 2);
  fs.writeFileSync(dataPath, outputJson);
}

main();
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Our script does the following:

Updating our schema

In order for this package to work, we need to update our schema.json file to take some extra values x-faker and x-cuid to know what to use and when:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "definitions": {
    "User": {
      "type": "object",
      "properties": {
        "id": {
          "type": "string",
          "x-cuid": "cuid"
        },
        "email": {
          "type": ["string", "null"],
          "x-faker": "internet.email"
        },
        "name": {
          "type": ["string", "null"],
          "x-faker": "name.findName"
        },
        "emailVerified": {
          "anyOf": [
            {
              "type": "string",
              "format": "date-time"
            },
            {
              "type": "null"
            }
          ]
        },
        "image": {
          "type": ["string", "null"],
          "x-faker": "image.avatar"
        }
      },
      "required": ["id", "email", "name", "emailVerified", "image", "role"],
      "additionalProperties": false,
      "description": "Model User"
    },
    "UserRole": {
      "type": "string",
      "enum": ["USER", "SUPERUSER"],
      "description": "Enums"
    },
    "Post": {
      "type": "object",
      "properties": {
        "id": {
          "type": "string",
          "x-cuid": "cuid"
        },
        "createdAt": {
          "type": "string",
          "format": "date-time"
        },
        "updatedAt": {
          "type": "string",
          "format": "date-time"
        },
        "title": {
          "type": "string"
        },
        "content": {
          "type": ["string", "null"]
        },
        "published": {
          "type": "boolean"
        },
        "authorId": {
          "type": "string"
        }
      },
      "required": [
        "id",
        "createdAt",
        "updatedAt",
        "title",
        "content",
        "published",
        "authorId"
      ],
      "additionalProperties": false,
      "description": "Model Post"
    }
  }
}
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Running the script

Finally, we can run node scripts/generate-data-from-json-schema.js to generate the data.

Once run successfully, we can see the output in __generated__/data.json:

{
  "User": {
    "id": "ckwn7ovf00000kvv5cg0kemkb",
    "email": "Karianne.Runolfsdottir0@gmail.com",
    "name": "Joyce Stamm",
    "emailVerified": null,
    "image": "https://cdn.fakercloud.com/avatars/gojeanyn_128.jpg"
  },
  "Post": {
    "id": "ckwn7ovfd0008kvv58ud5092m",
    "createdAt": "2005-08-29T14:00:00.0Z",
    "updatedAt": "1983-05-04T14:00:00.0Z",
    "title": "dolor",
    "content": null,
    "published": true,
    "authorId": "ckwn7ovf00000kvv5cg0kemkb"
  }
}
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Summary

Today's post demonstrated how to generate fake data based on the JSON Schema Faker library.

It also demonstrated how to extend the functionality to use our own libraries to generate the data.

Resources and further reading

Photo credit: wisniewski