transmute

No-inheritance transform streams.

Installation

transmute is available on npm.

npm install transmute

Usage

Transmute makes it easier to build transform streams avoiding inheritance in simple scenarios where it is not needed.

var fs = require('fs');
var transmute = require('transmute');

var lower = fs.createReadStream('lowercase.txt');
var upper = fs.createWriteStream('uppercase.txt');

var uppercase = transmute(function(chunk, enc, cb) {
  cb(null, chunk.toString().toUpperCase());
});

lower.pipe(uppercase).pipe(upper);

If options need to be provided (such as objectMode), transmute makes it easy to customize the transform stream. Here are all available options.

• tranform {Function} transform function
• flush {Function} flush function
• initialize {Function} construction hook function
• options {Object} options for Tranform construction
• readable {Object} options to merge with .readableState
• writable {Object} options to merge with .writableState

The following example illustrates objectMode when writing to the stream but the same also applies for readable.

var transmute = require('transmute');
var log = transmute({ options: { objectMode: true, hightWaterMark: 1 }});

var stringify = transmute({
    writable: { objectMode: true, highWaterMark: 1 }
  , transform: function(obj, enc, cb) {
      // avoid circular references
      try { var res = JSON.stringify(obj); }
      catch (ex) { return cb(ex); }
      cb(null, res + '\n');
    }
});

log.pipe(stringify).pipe(process.stdout);
log.write({ type: 'debug', msg: 'debug the universe' });
log.write({ type: 'info', msg: 'hello universe' });

If the same options apply for both sides of the transform then don't repeat yourself.

var nodebug = transmute({
    options: { objectMode: true, highWaterMark: 1 }
  , transform: function(obj, enc, cb) {
      if (obj.type === 'debug') return cb();
      cb(null, obj);
    }
});

log.pipe(nodebug).pipe(stringify).pipe(process.stdout);
log.write({ type: 'debug', msg: 'debug the universe' });
log.write({ type: 'info', msg: 'hello universe' });

If you want to provide a flush function to be invoked when the stream ends, that is easy too. Using everything we learned.

// examples/simple-logger.js
var transmute = require('transmute');

var log = transmute({
    options: { objectMode: true, highWaterMark: 1 }
  , transform: function(obj, enc, cb) {
      obj.timestamp = new Date();
      cb(null, obj);
    }
  , flush: function(cb) {
      var ended = {};
      ended.timestamp = new Date();
      ended.type = 'info';
      ended.msg = 'Log ended';
      this.push(ended);
      cb();
    }
});

var nodebug = transmute({
    options: { objectMode: true, highWaterMark: 1 }
  , transform: function(obj, enc, cb) {
      if (obj.type === 'debug') return cb();
      cb(null, obj);
    }
});

var display = transmute({
    writable: { objectMode: true, highWaterMark: 1 }
  , transform: function(obj, enc, cb) {
      cb(null, obj.timestamp.toUTCString() + ' [' + obj.type + '] ' + obj.msg + '\n');
    }
});

log.pipe(nodebug).pipe(display).pipe(process.stdout);
log.write({ type: 'info', msg: 'hello universe' });
log.write({ type: 'debug', msg: 'debug the universe' });
log.write({ type: 'info', msg: 'did you see the debug, thought not.' });
log.end();

Inspiration

transmute is certainly not the first to implement this idea. Here are some alternatives if you like:

• dominictarr/through - the original streams1 implementation
• rvagg/through2 - another streams2 implementation, but less granular control over readable/writable options

License

(The MIT License)

Copyright (c) 2012 Jake Luer [email protected] (http://alogicalparadox.com)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.