Hooking into Node.js stdout

gistfile1.js

var util = require('util')

function hook_stdout(callback) {
    var old_write = process.stdout.write

    process.stdout.write = (function(write) {
        return function(string, encoding, fd) {
            write.apply(process.stdout, arguments)
            callback(string, encoding, fd)
        }
    })(process.stdout.write)

    return function() {
        process.stdout.write = old_write
    }
}

console.log('a')
console.log('b')

var unhook = hook_stdout(function(string, encoding, fd) {
    util.debug('stdout: ' + util.inspect(string))
})

console.log('c')
console.log('d')

unhook()

console.log('e')
console.log('f')

Thanks @pguillory, I've used it on debug-logger

Genius! In my implementation, I created a new object to add my changes to:

var stdout = process.stdout;
var shadow = process.stdout = Object.create(process.stdout);
shadow.original = stdout;
shadow.write = function() {
    // stdout.original.write.apply(stdout, arguments); // not shown
    result.stdout += data;
};

(Which didn't work. Ignore me. ;P)

Thanks for this.

FYI. Commenting out this line mutes the log too.

This implementation redirects both stdout and stderr to a log file:

var log_file = require('fs').createWriteStream(__dirname + '/log.txt', {flags : 'w'})

function hook_stream(stream, callback) {
  var old_write = stream.write

  stream.write = (function(write) {
    return function(string, encoding, fd) {
      write.apply(stream, arguments)  // comments this line if you don't want output in the console
      callback(string, encoding, fd)
    }
  })(stream.write)

  return function() {
    stream.write = old_write
  }
}

console.log('a')
console.error('b')

var unhook_stdout = hook_stream(process.stdout, function(string, encoding, fd) {
  log_file.write(string, encoding)
})

var unhook_stderr = hook_stream(process.stderr, function(string, encoding, fd) {
  log_file.write(string, encoding)
})

console.log('c')
console.error('d')

unhook_stdout()
unhook_stderr()

console.log('e')
console.error('f')

It should print in the console

a
b
c
d
e
f

and in the log file:

c
d

This one works with AWS Lambda:

function hookStdout(callback) {
  const boundProcessStdout = process.stdout.write.bind(process.stdout)
  const boundProcessStderr = process.stderr.write.bind(process.stderr)

  process.stdout.write = (string, encoding, fd) => {
    boundProcessStdout(string, encoding, fd)
    callback(string, encoding, fd, false)
  }

  process.stderr.write = (string, encoding, fd) => {
    boundProcessStderr(string, encoding, fd)
    callback(string, encoding, fd, true)
  }

  return () => {
    process.stdout.write = boundProcessStdout
    process.stderr.write = boundProcessStderr
  }
}

thanks for this.

Thank you so much for this! I'm trying to make an advanced CLI for a nextjs app, but webpack uses stdout and messes up all of the formatting!