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Filter object streams

cop - modify object stream

The cop Node.js package offers a Transform stream to conventiently cop data from object streams.

Use require('cop') to get a function that returns a Transform stream, configured in object mode, to which you can write arbitrary data. This constructor function takes either a String or a function as its sole argument.

Specify a String to select a property, and the cop stream will emit the values of matching properties for all objects written to it. If an object doesn't have a property with this name, or the property's value is null, the stream doesn't end, but just skips the object and moves on the next.

  • key The name of the property of which to emit the value.
var cop = require('cop')
var stream = require('stream')
var objs = [
  { name: 'Moe' },
  { name: 'Larry' },
  { name: 'Curly' }
var stooges = new stream.Readable({ objectMode: true })
stooges._read = function () {
  stooges.push(objs.shift() || null)

To apply custom transformations to each object, you can supply your own synchronous map function, which is applied with each object and is expected to return an arbitrary object to be emitted by the cop stream. If you want to skip the current object, you can return null or undefinedthe stream won't stop. For asynchronous work, of course, you should write your own stream.

  • map A function which is applied to each object written to the stream.

The following use case, streaming file entries with fstream and emitting just the filenames, filtering out directories; is the origin of this module. The filtering happens internally btw, sorry about that. But you get the point, you can do anything you want in the map function.

var cop = require('cop')
var fstream = require('fstream')
function map (obj) {
  return obj ? obj['path'] + '\n' : undefined
fstream.Reader({ path: __dirname })

With npm do:

$ npm install cop

MIT License