fetch.md

using gtfs-utils in a browser

You can use gtfs-utils with a remote GTFS feed efficiently if they are

• hosted as individual files (e.g. on an HTTP server), and
• sorted in the way that gtfs-utils needs them.

This way, you can process the files incrementally while they are being downloaded chunk by chunk. The following example shows how to use the fetch Web API for that.

Because a fetch response body is a Web Streams API ReadableStream and gtfs-utils/read-csv needs a Node.js stream.Readable, adapting code is necessary.

const {Readable} = require('stream')
const readCsv = require('gtfs-utils/read-csv')
const computeStopovers = require('gtfs-utils/compute-stopovers')

// convert a fetch response body (Web Streams API) into a Node.js stream.Readable
const readableStreamFromResponseBody = (resBody) => {
	const reader = resBody.getReader()
	let bytes = 0
	const readable = new Readable({
		read: () => {
			reader.read()
			.then(({done, value}) => {
				if (done) readable.push(null)
				else {
					bytes += value.buffer.byteLength
					const buf = Buffer.from(value.buffer)
					readable.push(buf)
				}
			})
			.catch(err => readable.destroy(err))
		},
		destroy: (err, cb) => {
			reader.cancel(err && err.message || null)
			.then(() => cb(), () => cb())
		},
	})
	return readable
}

const readFile = async (name) => {
	const res = await fetch(`/${name}.txt`, {
		mode: 'cors',
		headers: {
			'accept': 'text/plain',
		},
	})
	if (!res.ok) { // non-2xx HTTP response
		const err = new Error(`${res.statusText} ${res.url}`)
		// gtfs-utils uses err.statusCode to ignore if optional files
		// (e.g. frequencies.txt) are missing.
		err.statusCode = res.status
		err.res = res
		throw err
	}

	const readable = readableStreamFromResponseBody(res.body)
	return readCsv(readable)
}

const stopovers = computeStopovers(readFile, 'Europe/Berlin')
for await (const stopover of stopovers) console.log(stopover)