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, I've created a use case example for when one uses cluster and wishes to separate per-process output.

module.exports = function(mediator) {

    var fs          = require('fs'),
        path        = require('path'),
        config      = require('../../config'),
        env         = process.env,
        stdout      = process.stdout,
        stderr      = process.stderr,
        workerId    = env.NODE_WORKER_ID || env.NODE_UNIQUE_ID,
        logDir      = config.debug.logDir,
        stdoutFile  = path.resolve(logDir, 'worker#' + workerId + '.log'),
        stderrFile  = path.resolve(logDir, 'worker#' + workerId + '.err.log');

    //run prior to boot ready
    mediator.once('boot.init', function() {

        //create a new stdout file stream
        var stdoutFS = fs.createWriteStream(stdoutFile, {
            encoding: 'utf8',
            flags   : 'a+'
        });

        //create a new stderr file stream
        var stderrFS = fs.createWriteStream(stderrFile, {
            encoding: 'utf8',
            flags   : 'a+'
        });

        //pipe stdout to a worker file
        var unhookStdout = hook_writestream(stdout, function(string, encoding, fd) {
            stdoutFS.write(string, encoding || 'utf8');
        });
        console.log('\n\nPrepared new stdout hook to worker file.');

        //pipe stderr to a worker file
        var unhookStderr = hook_writestream(stderr, function(string, encoding, fd) {
            stderrFS.write(string, encoding || 'utf8');
        });
        console.log('\n\nPrepared new stderr hook to worker file.');

        //unhook when things go wrong
        stdoutFS.once('close', function() {
            unhookStdout();
            console.log('Unhooked stdout.');
        });
        stdoutFS.once('error', function(err) {
            unhookStdout();
            console.error('Error: Unhooked stdout due to error %j.', err);
        });
        stderrFS.once('close', function() {
            unhookStderr();
            console.log('Unhooked stderr.');
        });
        stderrFS.once('error', function(err) {
            unhookStderr();
            console.error('Error: Unhooked stderr due to error %j.', err);
        });

    });

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

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

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

};

Thanks for this :)

Thanks!

Crystallized it a bit further here:
https://github.com/mikermcneil/fixture-stdout

Another approach with knobs
https://gist.github.com/stringparser/b539b8cfd5769542037d

Awesome magic here. 😄 Thanks!

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!