raine / hook-stream

Hook into writes of a stream

hook-stream

Allows listening to a given stream's writes through a new, readable stream.

Useful for listening to a 3rd party library's writes to stdout or stderr.

install

npm install --save hook-stream

API

hookStream(stream: Stream, options?: Object): [unhook: Function, readable: Stream]

Takes a stream to hook into and optional options object.

Returns a tuple of unhook function and a newly created readable stream that emits writes to stream.

example

Direct writes to stderr to stdout.

const hookStream = require('hook-stream')
const map = require('through2-map')
const treis = require('treis')

const [ unhook, stream ] = hookStream(process.stderr)

stream
  .pipe(map((x) => `STDOUT ${x}`))
  .pipe(process.stdout)

console.error('foo')
process.stderr.write('bar')
// treis prints function input and output to stderr
treis(x => x)(1)

unhook()

output

% node examples/1.js
STDOUT foo
foo
STDOUT bar
bar
STDOUT λ1 x: 1
λ1 x: 1
STDOUT λ1 => 1
λ1 => 1

caveat

This method does not work if a third party lib saves a reference to "original" stream.write (e.g. process.stderr.write) before hookStream is called.

// somewhere else, maybe in some lib's code
const log = process.stderr.write.bind(process.stderr)

// .....

// won't work because hookStream replaces process.stderr.write with its own
// function, but log already has reference to the original
const [ unhook, stream ] = hookStream(process.stderr)