How can I write a simple gulp pipe function?

Question

I've been trying for a day to write two pipe functions, one that compiles less files and another one that concats these files. I want to learn how to write transform streams/pipes for more complex plugins.

So I want to know how to read data from another pipe, and how to alter that data and send it to the next pipe. This is what I have so far:

 gulp.src(sources)
   .pipe(through.obj(function (chunk, enc, cb) {

     var t = this;
     // console.log("chunk", chunk.path);
     fs.readFile(chunk.path, enc, function (err,data) {
       if (err) { cb(err); }

       less.render(data, {
         filename : chunk.path,
         sourceMap : {
           sourceMapRootpath : true
         }
       })
       .then(function (outputCss) {
          // console.log("less result",outputCss);
          t.push(chunk);// or this.push(outputCss) same result
          cb();
       });

     });

   }))
   .pipe(through.obj(function (chunk, enc, cb) {
     console.log("chunk", chunk.path); // not event getting called.
     cb();
   }))

I can't get the outputCSS for each file in the second pipe. How can I send it?

Answer 1

Well, you don't need to use fs here, you already got the stream of file (here your chunk).

Another point, you're not sending back to the pipe the files, so I guess that's why nothing is called on your second one.

var through = require('through2')

gulp.src(sources)
  .pipe(through.obj(function (chunk, enc, cb) {
    console.log('chunk', chunk.path) // this should log now
    cb(null, chunk)
  }))

In ES2015:

import through from 'through2'

gulp.src(sources)
  .pipe(through.obj((chunk, enc, cb) => cb(null, chunk)))

And for your specific example:

.pipe(through.obj(function (file, enc, cb) {
  less.render(file.contents, { filename: file.path, ... }) // add other options
    .then(function (res) {
      file.contents = new Buffer(res.css)
      cb(null, file)
    })
}))

This is still pretty basic, I don't check for errors, if it's not a stream and so on, but this should give you some hint on what you've missed.