@api-ts/io-ts-http

Runtime types for (de)serializing HTTP requests from both the client and server side

Contents

- @api-ts/io-ts-http
- Contents
- Preface
- Introduction
- Overview
- Example
- apiSpec
- httpRoute
- path
- method
- request
- response
- httpRequest
- params
- query
- headers
- body
- Decoding an httpRequest
- Documentation

Preface

This package extends io-ts with functionality useful
for typing HTTP requests. Start there for base knowledge required to use this package.

Introduction

io-ts-http is the definition language for api-ts specifications, which define the API
contract for a web sever to an arbitrary degree of precision. Web servers can use the
io-ts-http spec to parse HTTP requests at runtime, and encode HTTP responses. Clients
can use the io-ts-http spec to enforce API compatibility at compile time, and to encode
requests to the server and decode responses.

Overview

The primary function in this library is httpRequest. You can use this to build codecs
which can parse a generic HTTP request into a more refined type. The generic HTTP
request should conform to the following interface:

``typescript
interface GenericHttpRequest {
params: {
[K: string]: string;
};
query: {
[K: string]: string | string[];
};
headers: {
[K: string]: string;
};
body?: unknown;
}
`

Here, params represents the path parameters and query is minimally-parsed query
string parameters (basically just the results of splitting up the query string and
urlDecoding the values). The httpRequest function can be combined with codecs from
io-ts to build a combined codec that is able to validate, parse, and encode these
generic HTTP requests into a more refined object. For example:

`typescript
import { httpRequest, optional } from '@api-ts/io-ts-http';
import { DateFromISOString, NumberFromString } from 'io-ts-types';

const ExampleHttpRequest = httpRequest({
query: {
id: NumberFromString,
time: optional(DateFromISOString),
},
});
`

This builds a codec that can be given an arbitrary HTTP request and will ensure that it
contains an id parameter, and also optionally will check for a time parameter, and
if it is present, validate and parse it to a Date. If decoding succeeds, then the
resulting value's type will be:

`typescript
type ExampleDecodedResult = {
id: number;
time?: Date;
};
`

This type is properly inferred by TypeScript and can be used in destructuring like so:

`typescript
import { pipe } from 'fp-ts/function';
import * as E from 'fp-ts/Either';

const { id, time } = pipe(
ExampleHttpRequest.decode(request),
E.getOrElseW((decodeErrors) => {
someErrorHandler(decodeErrors);
}),
);
`

to get request argument validation and parsing as a one-liner. These codecs can also be
used from the client-side to get the type safety around making outgoing requests. An API
client could hypothetically have a method like:

`typescript
apiClient.request(route, ExampleHttpRequest, {
id: 1337,
time: new Date(),
});
`

If both the server and client use the same codec for the request, then it becomes
possible to encode the API contract (or at least as much of it that is possible to
express in the type system) and therefore someone calling the API can be confident that
the server will correctly interpret a request if the arguments typecheck.

Example

Let's define the api-ts spec for a hypothetical message-user service. The conventional
top-level export is an
apiSpec
value; for example:

$3

`typescript
import { apiSpec } from '@api-ts/io-ts-http';

import { GetMessage, CreateMessage } from './routes/message';
import { GetUser, CreateUser, PatchUser, UpdateUser, DeleteUser } from './routes/user';

/**
* message-user service
*
* @version 1.0.0
*/
export const API = apiSpec({
'api.v1.message': {
get: GetMessage,
post: CreateMessage,
},
'api.v1.user': {
get: GetUser,
post: CreateUser,
put: UpdateUser,
delete: DeleteUser,
patch: PatchUser,
},
});
`

The apiSpec is imported, along with some named httpRoutes ({Get|Create}Message,
and {Get|Create|Update|Delete}User) which we'll discuss below.

> Currently, if you add the @version JSDoc tag to the exported API spec, it will be
> used as the API version when generating an OpenAPI schema. Support for other tags
> may be added in the future.

The top-level export for message-user-types is API, which we define as an apiSpec
with two endpoints api/v1/message and api/v1/user. The api/v1/message endpoint
responds to GET and POST verbs while the second reponds to GET, POST, PUT, and
DELETE verbs using httpRoutes defined in ./routes/message. The following are the
httpRoutes defined in ./routes/message.

$3

`typescript
import * as t from 'io-ts';
import { httpRoute, httpRequest } from '@api-ts/io-ts-http';

export const GetMessage = httpRoute({
path: '/message/{id}',
method: 'GET',
request: httpRequest({
params: {
id: t.string,
},
}),
response: {
200: t.type({
id: t.string,
message: t.string,
}),
404: t.type({
error: t.string,
}),
},
});

export const CreateMessage = httpRoute({
path: '/message',
method: 'POST',
request: httpRequest({
body: {
message: t.string,
},
}),
response: {
200: t.type({
id: t.string,
message: t.string,
}),
404: t.type({
error: t.string,
}),
},
});
`

The first import is the io-ts package. It's usually imported as t for use in
describing the types of data properties. Again, review
io-ts documentation for more context on how to use it
and this package.

Then httpRoute and httpRequest are imported. We'll review the
httpRequest below, but first, let's review the GetMessage
httpRoute.

`typescript
export const GetMessage = httpRoute({
path: '/message/{id}',
method: 'GET',
request: httpRequest({
params: {
id: t.string,
},
}),
response: {
200: t.type({
id: t.string,
message: t.string,
}),
404: t.type({
error: t.string,
}),
},
});
`

httpRoutes
apiSpecs use
httpRoutes
to define the path, method, request and response of a route.

#### path

The route's path along with possible path variables. You should surround variables
with brackets like {name}, and are to the request codec under the params property.

#### method

The route's method is the
HTTP request method to use
for that route. In our GetMessage example, the method is GET, while in our
PostMessage example, the method is POST.

#### request

The route's request is the output of the httpRequest function. This will be
described below.

#### response

The route's response describes the possible
HTTP responses the route can
produce. The key-value pairs of the response object are an HTTP status code followed
by the io-ts type of the response body. In our GetMessage example, a 200 status
response yields a payload of a JSON object with two properties, message which is a
string and id which is also a string, and a 404 yeilds a payload of a JSON
object with a single property error which is a String.

$3

Use httpRequest to build the expected type that you pass in a request to the route. In
our example GetMessage

`typescript
export const GetMessage = httpRoute({
path: '/message/{id}',
method: 'GET',
request: httpRequest({
params: {
id: t.string,
},
}),
// ...
});
`

httpRequests have a total of 4 optional properties: params (shown in the example),
query, headers, and body.

#### params

params is an object representing the types of path parameters in a URL. Any URL
parameters in the path property of an httpRoute must be accounted for in the
params property of the httpRequest. Our request has a single URL parameter it is
expecting, id. This is the type of this parameter is captured in the params object
of our httpRequest.

#### query

query is the object representing the values passed in via query parameters at the end
of a URL. The following example uses a new route, GetMessages, to our API that
searches messages related to a specific author:

`typescript
export const GetMessages = httpRoute({
path: '/messages',
method: 'GET',
request: httpRequest({
query: {
author: t.string,
},
}),
// ...
});
`

#### headers

headers is an object representing the types of the
HTTP headers passed in with
a request.

#### body

body is an object representing the type of the
HTTP body of a
request. Often this is a JSON object. The CreateMessage httpRoute in our example
uses the body property:

`typescript
export const CreateMessage = httpRoute({
path: '/message',
method: 'POST',
request: httpRequest({
body: {
message: t.string,
},
}),
// ...
});
`

#### Decoding an httpRequest

When you decode httpRequests using io-ts helpers, the properties of the request are
flattened like this:

`typescript
import { DateFromISOString, NumberFromString } from 'io-ts-types';

// build an httpRequest with one parameter id and a body with content and a timestamp
const Request = httpRequest({
params: {
id: NumberFromString,
},
body: {
content: t.string,
timestamp: DateFromISOString,
},
});

// use io-ts to get the type of the Request
type Request = t.TypeOf;

// same as
type Request = {
id: number;
content: string;
timestamp: Date;
};
``

Documentation

- API Reference