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Ketting is a Hypermedia client for javascript
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Ketting - A hypermedia client for javascript


The Ketting library is an attempt at creating a 'generic' hypermedia client, it supports an opinionated set of modern features REST services might have.

The library supports HAL, JSON:API, Siren, Collection+JSON, Web Linking (HTTP Link Header) and HTML5 links. It uses the Fetch API and works both in the browsers and in node.js.


const ketting = new Ketting('');

// Follow a link with rel="author". This could be a HTML5 `<link>`, a
// HAL `_links` or a HTTP `Link:`.
const author = await ketting.follow('author');

// Grab the current state
const authorState = await author.get();

// Change the firstName property of the object. Note that this assumes JSON.
authorState.firstName = 'Evert';

// Save the new state
await author.put(authorState);


Notable Features

Ketting is a library that sits on top of a Fetch API to provide a RESTful interface and make it easier to follow REST best practices more strictly.

It provides some useful abstractions that make it easier to work with true hypermedia / HATEAOS servers. It currently parses HAL and has a deep understanding of links and embedded resources. There's also support for parsing and following links from HTML documents, and it understands the HTTP Link: header.

Using this library it becomes very easy to follow links from a single bookmark, and discover resources and features on the server.

Supported formats:

  • HAL
  • Siren
  • HTML - Can automatically follow <link> and <a> element with rel= attributes.
  • HTTP Link header - automatically registers as links regardless of format.
  • JSON:API - Understands the links object and registers collection members as item relationships.
  • application/problem+json - Will extract useful information from the standard problem object and embed them in exception objects.

Other features:

Following links

One core tenet of building a good REST service, is that URIs should be discovered, not hardcoded in an application. It's for this reason that the emphasis in this library is not on URIs (like most libraries) but on relation-types (the rel) and links.

Generally when interacting with a REST service, you'll want to only hardcode a single URI (a bookmark) and discover all the other APIs from there on.

For example, consider that there is a some API at This API has a link to an API for news articles (rel="articleCollection"), which has a link for creating a new article (rel="new"). When POSTing on that URI, the API returns 201 Created along with a Location header pointing to the new article. On this location, a new rel="author" appears automatically, pointing to the person that created the article.

This is how that interaction might look like:

const ketting = new Ketting('');
const createArticle = await ketting.follow('articleCollection').follow('new'); // chained follow

const newArticle = await{ title: 'Hello world' });
const author = await newArticle.follow('author');

// Output author information
console.log(await author.get());

More details can be found in the Getting started docs and Following Links.

Embedded resources

Embedded resources are a HAL feature. In situations when you are modeling a 'collection' of resources, in HAL you should generally just create links to all the items in the collection. However, if a client wants to fetch all these items, this can result in a lot of HTTP requests. HAL uses _embedded to work around this. Using _embedded a user can effectively tell the HAL client about the links in the collection and immediately send along the contents of those resources, thus avoiding the overhead.

Ketting understands _embedded and completely abstracts them away. If you use Ketting with a HAL server, you can therefore completely ignore them.

For example, given a collection resource with many resources that hal the relationshiptype item, you might use the following API:

const ketting = new Ketting('');
const articleCollection = await ketting.follow('articleCollection');

const items = await someCollection.followAll('item');

for (const item of items) {
   console.log(await item.get());

Given the last example, if the server did not use embedding, it will result in a HTTP GET request for every item in the collection.

If the server did use embedding, there will only be 1 GET request.

A major advantage of this, is that it allows a server to be upgradable. Hot paths might be optimized using embedding, and the client seamlessly adjusts to the new information.

Further reading:

Automatically parsing problem+json

If your server emits application/problem+json documents (RFC7807) on HTTP errors, the library will automatically extract the information from that object, and also provide a better exception message (if the title property is provided).

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