Basic pattern for creating a custom Transform stream for use with gulp tasks.

custom-task.js

var gulp = require('gulp');
var myTransform = require('./myTransform');

gulp.task('foobar', function (){
    return gulp.src("foobar.js")
        .pipe(myTransform())
        .pipe(gulp.dest('.'));
});

myTransform.js

var through = require('through2');

module.exports = function () {
  // return a `through2` stream for `pipe()` compatibility at the node level
  return through.obj(function (vinylFile, encoding, callback) {
    // 1. clone new vinyl file for manipulation
    // (See https://github.com/wearefractal/vinyl for vinyl attributes and functions)
    var transformedFile = vinylFile.clone();

    // 2. set new contents
    // * contents can only be a Buffer, Stream, or null
    // * This allows us to modify the vinyl file in memory and prevents the need to write back to the file system.
    transformedFile.contents = new Buffer("whatever");

    // 3. pass along transformed file for use in next `pipe()`
    callback(null, transformedFile);
  });
}//myTransform()