@rdfjs/fetch-lite

@rdfjs/fetch-lite

Wrapper for fetch to simplify sending and retrieving RDF data.

This is a light version of the @rdfjs/fetch package, without the @rdfjs/formats-common dependency. It is useful when you want to make a build for the browser with a reduced set of parsers and serializers.

The formats options is required for this package. See also the @rdfjs/fetch documentation.

Since version 3.0, this packages is ESM only. Check version 2.x if you are looking for a CommonJS package.

Usage

The package exports a fetch function which wraps the request and response object for on-the-fly RDF quad processing. The function accepts the same parameters as fetch and some additional options. It also provides extra methods.

Options

The options object accepts the following additional parameters:

• formats: A formats-common-compatible object which contains a set of parsers and serializers. This parameter is required.
• factory: If given, the factory will be used to create a Dataset when dataset() is called. If the parameter is not given, the dataset() method will not be attached to the response.
• fetch: An alternative fetch implementation. By default nodeify-fetch will be used.

The following options influence the logic of RDF quad processing:

• headers.accept: The accept header field will be automatically set base on the list of available parsers from the formats object. If it's already set it will not be overwritten. This can be useful when only a subset of the available parsers should be used.
• headers.content-type: When the request has a body, this header field will be automatically set to use matching media type for the corresponding serializer. By setting this field manually a specific serializer can be enforced.
• body: If the request should send quads, the quads must be given either as a stream or as an iterable. Iterables will be converted to streams before they are handed over to the serializer.

Response

The following methods are attached to the standard fetch response object:

• async quadStream(): This method returns the quads of the response as stream. The parser is selected based on the content type header field.
• async dataset(): This method uses the quadStream() method to parse the content and will pipe it into a dataset, which is also the return value. This method is only available when the factory option is given.

The Content-Type header of the response can be changed or set before calling quadStream() or dataset(). That allows enforcing a specific parser or can be used to fix a lacking header.

Example

This example fetches data from a resource on Wikidata. The stream API is used to process all quads. For all rdfs:label quads of the defined entity, the object language and value will be written to the console.

const formats = require('@rdfjs/formats-common')
const fetch = require('@rdfjs/fetch-lite')

const entity = 'http://www.wikidata.org/entity/Q2'
const label = 'http://www.w3.org/2000/01/rdf-schema#label'

fetch('https://www.wikidata.org/wiki/Special:EntityData/Q2.ttl', { formats })
  .then(res => res.quadStream())
  .then(quadStream => {
    return new Promise(resolve => {
      quadStream.on('end', resolve)

      quadStream.on('data', quad => {
        if (quad.subject.value === entity && quad.predicate.value === label) {
          console.log(`${quad.object.language}: ${quad.object.value}`)
        }
      })
    })
  })
  .catch(err => console.error(err))