Practical Example

Just want to try it by yourself?

Skip to the Quickstart or head over to our Migration Guide to migrate your existing project to Kita.

No matter how good we are at explaining, nothing is better than an example to prove to you how powerful Kita is in just a few lines of code.

Below is the simplest Fastify server you’ve ever seen:

src/index.ts

import { Kita } from '@kitajs/runtime';
import fastify from 'fastify';

fastify().register(Kita).listen({ port: 1227 });

Just by creating a file at src/routes/index.ts and exporting a get function, you already have a GET route registered in your Fastify server.

src/routes/index.ts

export function get() {
  return { hello: 'world' };
}

Running the Server

As Kita runs at compile time, you need to run the kita build command before running or compiling your code:

Terminal

npx kita build

Now, run your server:

Terminal

$ npx ts-node src/index.ts
# npx tsc && node dist/index.js is also valid

Testing the Route

Done! Your server is already running and you can access the GET route at http://localhost:1227.

Terminal

$ curl http://localhost:1227
{ "hello": "world" }

But wait, there’s more! Kita, along with Scalar, Kita generates a graphical interface for you to visualize, edit, and test your API:

Click here to launch a STATIC DEMO of the Scalar Editor

Or view the generated OpenAPI json

{
  "openapi": "3.1.0",
  "info": {
    "title": "API Reference",
    "description": "Powered by [Scalar](https://scalar.com/) & Generated by [KitaJS](https://kitajs.org/)",
    "version": "x.x.x"
  },
  "components": {
    "schemas": {
      "GetIndexResponse": {
        "additionalProperties": false,
        "properties": { "hello": { "type": "string" } },
        "required": ["hello"],
        "type": "object"
      }
    }
  },
  "paths": {
    "/": {
      "get": {
        "operationId": "getIndex",
        "responses": {
          "2XX": {
            "description": "Default Response",
            "content": {
              "application/json": {
                "schema": { "$ref": "#/components/schemas/GetIndexResponse" }
              }
            }
          }
        }
      }
    }
  }
}

Let’s change something!

Life isn’t perfect, and eventually, you’ll receive new specifications to implement… Let’s do this? Notice how simple it is to transform your routes into something more complex.

1. We’ll change the method from GET to POST.

2. We’ll add a Body containing a name: string field (which must have a minimum of 5 characters) and an age: number field (which must be greater than 18).

3. We’ll also use the Authorization header to validate whether the user has permission to create a new user.

4. We’ll also use a Query of ?fail=<boolean> which has a default value of false.

5. Lets also return the newly created user with an automatically generated id.

6. We can also return a Forbidden status if the Authorization is invalid and a 400 error if the request comes with ?fail=true.

7. It’s always recommended to add descriptions for the fields, so we’ll do that too.

The final result should look something like this:

src/routes/index.ts

import type { HttpErrors } from '@fastify/sensible';
import type { Body, Header, Query } from '@kitajs/runtime';

interface PostBody {
  /** @minLength 5 */
  name: string;

  /** @minimum 18 */
  age: number;
}

/**
 * Creates a new user
 *
 * @operationId createUser
 */
export function post(
  body: Body<PostBody>,
  fail: Query<boolean> = false,
  authorization: Header<'authorization'>,
  errors: HttpErrors
) {
  if (authorization !== 'please let me in') {
    throw errors.forbidden('Invalid Authorization');
  }

  if (fail) {
    throw errors.badRequest('You asked for it');
  }

  return {
    id: Math.floor(Math.random() * 1000),
    name: body.name,
    age: body.age
  };
}

Yes, you read it right! Just by using typing and annotations, your route is already documented, being validated at runtime, and secure against attacks like Prototype Pollution.

What happened here?

I know you might not fully understand all the power that is now in your hands. Let me break down everything you just did:

• By changing the function name from get to post, the route is automatically changed to POST.

• By adding a body parameter of type Body<PostBody>, Kita understands that the request body needs to be passed there, as well as Kita reads the typing of the PostBody, validates the body at runtime to ensure it’s in the format of PostBody, and generates the necessary documentation.

• The PostBody interface has only 2 fields, name and age, so ONLY these fields will be present at runtime, extra fields present in the JSON will be ignored.

• By annotating with @minLength 5 and @minimum 18 on the name and age fields, respectively, Kita ensures that these conditions will be respected at runtime.

• By adding a fail parameter of type Query<boolean> and the default value of false, Kita understands that it needs to extract the value from the query string (?fail=true) and that if not specified, it should use the value false.

• By adding an authorization parameter of type Header<'authorization'>, Kita understands that it needs to extract the value from the Authorization header and that this value is mandatory.

• Inside the function, by throwing any errors coming from the errors parameter, Kita understands that it’s an HTTP error and will return the correct status as well as document it in OpenAPI.

• By returning an object with the id, name, and age fields, Kita understands that this is the return of the function and generates the necessary documentation and validation.

  Yes, if you return an object different from the specified typing, Kita will throw an error at runtime because your clients expect a different signature than what is being returned.

Result

First, you need to run the kita build again to re-perform the static analysis of your code and regenerate the necessary files:

Terminal

$ npx kita build
$ npx ts-node src/index.ts

After running the server, you can already access the POST route.

Since this is just an practical (non hosted) example, see the interface generated by Scalar:

Click here to launch a STATIC DEMO of the Scalar Editor

Or view the generated OpenAPI json

{
  "openapi": "3.1.0",
  "info": {
    "title": "API Reference",
    "description": "Powered by [Scalar](https://scalar.com/) & Generated by [KitaJS](https://kitajs.org/)",
    "version": "x.x.x"
  },
  "components": {
    "schemas": {
      "HttpError": {
        "type": "object",
        "properties": {
          "statusCode": { "type": "number" },
          "code": { "type": "string" },
          "error": { "type": "string" },
          "message": { "type": "string" }
        }
      },
      "CreateUserResponse": {
        "additionalProperties": false,
        "properties": {
          "age": { "type": "number" },
          "id": { "type": "number" },
          "name": { "type": "string" }
        },
        "required": ["id", "name", "age"],
        "type": "object"
      }
    }
  },
  "paths": {
    "/": {
      "post": {
        "operationId": "createUser",
        "description": "Creates a new user",
        "requestBody": {
          "content": {
            "application/json": {
              "schema": {
                "additionalProperties": false,
                "properties": {
                  "age": { "minimum": 18, "type": "number" },
                  "name": { "minLength": 5, "type": "string" }
                },
                "required": ["name", "age"],
                "type": "object"
              }
            }
          },
          "required": true
        },
        "parameters": [
          {
            "schema": { "type": "boolean" },
            "in": "query",
            "name": "fail",
            "required": false
          },
          {
            "schema": { "type": "string" },
            "in": "header",
            "name": "authorization",
            "required": true
          }
        ],
        "responses": {
          "400": {
            "description": "Default Response",
            "content": {
              "application/json": {
                "schema": { "$ref": "#/components/schemas/HttpError" }
              }
            }
          },
          "403": {
            "description": "Default Response",
            "content": {
              "application/json": {
                "schema": { "$ref": "#/components/schemas/HttpError" }
              }
            }
          },
          "2XX": {
            "description": "Default Response",
            "content": {
              "application/json": {
                "schema": { "$ref": "#/components/schemas/CreateUserResponse" }
              }
            }
          }
        }
      }
    }
  }
}

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