stream-data-converter v0.1.4

This library provides tools for stream-processing large, "iterable" data files (e.g. text, CSV, DSV, JSONP, array-like JSON, GeoJSON, Shapefile) that cannot be loaded into memory all at once.

It is best for tasks that can be performed independently on each iteration, such as:

• Inspect data and generate column-wise statistics
• Output a subset of the input data based on a filter
• Convert input data to another format
• Download and save data from remote API in batches

Install

npm install stream-data-converter

Usage

The core exports implement Node.js' stream.Transform class.

const {JSONReader, Transform, CSVWriter} = require('stream-data-converter');

/*
 * input.geojson
   {"type": "FeatureCollection", "features: [
     {"type":"Feature","properties":{"COUNT":1,"DATESTR":"2020-01-01 16:37:03Z"},"geometry":{"type":"Point","coordinates":[-122.45,37.78]}},
     ...
   ]}

 * output.tsv
   count  timestamp longitude latitude
   1  1577896623  -122.45 37.78
   ...
 */

fs.createReadStream('path/to/input.geojson')
  .pipe(new JSONReader())
  .pipe(new Transform(processFeature))
  .pipe(new CSVWriter({delimiter: '\t'}))
  .pipe(fs.createWriteStream('path/to/output.tsv'));

function processFeature(feature, index) {
  if (feature.properties.COUNT === 0) {
    return null; // discard
  }
  return {
    count: feature.properties.COUNT,
    timestamp: Date(feature.properties.DATESTR) / 1000,
    longitude: feature.geometry.coordinates[0],
    latitude: feature.geometry.coordinates[1]
  };
}

API Reference

Readers

The reader classes take a text-based input and ouput a semantically parsed iterable down the stream.

LineReader

Outputs an iterable where each item is a line of text.

const {LineReader} = require('stream-data-converter');
const reader = new LineReader(options);

Options

• id (String, optional): identifier of this reader instance.
• delimiter (String, optional): delimiter (line break) of the input. Default '\n'.
• skipEmptyLines (Boolean, optional): if true, empty lines are omitted. Default true.

CSVReader

Parses csv/dsv formatted input and outputs an iterable where each item is a JSON object describing a row.

const {CSVReader} = require('stream-data-converter');
const reader = new CSVReader(options);

Options

• id (String, optional): identifier of this reader instance.
• All options that are supported by Papaparse.parse. The following defaults are used instead of the original default config:
  • header: true
  • dynamicTyping: true
  • skipEmptyLines: true
  • newline: '\n'

JSONReader

Parses JSON input and outputs an iterable where each item is an element in the first object array (array of objects, in the form of [{}]).

If the input JSON is an array, then each iteration returns an item in this array.

If the input JSON is a nested object, then the reader looks for the first object array that it encounters, and returns its element one for each iteration. For example, a typical GeoJSON is in the shape of {type: 'FeatureCollection', features: [...]}, and this reader will iterate through each feature in the features array.

const {JSONReader} = require('stream-data-converter');
const reader = new JSONReader(options);

Options:

• id (String, optional): identifier of this reader instance.
• metadata (Boolean, optional): If false (default), just the array element itself is returned. If true, the element is wrapped an object in the shape of {type, data}:
  • type: 'arrayitem': data is an element of the object array
  • type: 'container': data is the outer JSON object, with the iterated object array empty.

Transform

Transforms an iterable from upstream to another iterable. Usually consumes the output of a Reader.

For each incoming iteration, Transform calls a callback function. This callback is expected to return one of the following:

• null or undefined: ignore this item
• Buffer or Uint8Array: push directly to the output iterable
• Object or string: serialize and push a single item to the output iterable
• Array: push multiple items to the output iterable

const {Transform} = require('stream-data-converter');
const transform = new Transform(options);

Options:

• If options is a function, it is used as the iteration callback.
• If options is an object, it can contain the following fields:
  • transform (Function): the iteration callback.
  • id (String, optional): identifier of this transform instance.
  • inputType (String, optional): json or text. Force the incoming item to be formatted. Default 'auto'.
  • maxConcurrency (Number, optional): if transform is an async function, use this option to limit how many can be running concurrently.
  • minTime (Number, optional): if transform is an async function, use this option to throttle the calls. Default 0 (no throttling).

Writers

Writers consume an iterable and output a formatted text stream that can be written to a file.

LineWriter

Writes each item to a line of text.

const {LineWriter} = require('stream-data-converter');
const writer = new LineWriter(options);

Options:

• id (String, optional): identifier of this writer instance.
• delimiter (String, optional): delimiter (line break) of the output. Default \n.
• endEmptyLine (Boolean, optional): if true, always append an empty line at the end. Default true.

CSVWriter

Writes each item to a row in csv/dsv formatted text.

const {CSVWriter} = require('stream-data-converter');
const writer = new CSVWriter(options);

Options:

• id (String, optional): identifier of this writer instance.
• All options that are supported by Papaparse.unparse. The following defaults are used instead of the original default config:
  • newline: '\n'
• batchSize: The number of items to encode in a batch. Default 100.

JSONWriter

Writes each item to a array elemnt in JSON formatted text.

const {JSONWriter} = require('stream-data-converter');
const writer = new JSONWriter(options);

Options:

• id (String, optional): identifier of this writer instance.
• container (Object, optional): the container object. Default [].
• path (String, optional): path to the parent array in the container object, joined by .. Default null.

GeoJSONWriter

Writes each item as a feature in GeoJSON FeatureCollection.

const {GeoJSONWriter} = require('stream-data-converter');
const writer = new GeoJSONWriter();
// This is equivalent of
const writer = new JSONWriter({
  container: {type: 'FeatureCollection', features: []},
  path: 'features'
});

Helper Utilities

logProgress

Logs streaming progress to the console. Useful when the process takes a long time to run.

const {JSONReader, Transform, GeoJSONWriter, logProgress} = require('stream-data-converter');

const reader = new JSONReader();
// Only keep features in California
const transform = new Transform(f => f.properties.state === 'CA' ? f : null);
const writer = new GeoJSONWriter();

logProgress(reader, transform, writer);

fs.createReadStream('path/to/input.geojson')
  .pipe(reader)
  .pipe(transform)
  .pipe(writer)
  .pipe(fs.createWriteStream('path/to/output.geojson'));

// The console will print a growing counter that looks like this:
// jsonReader: 9999 | transform: 9999 | jsonWriter: 7213