ast-pipeline

Seamlessly pipe between text transform streams and AST transforms.

Say, for example, you have a source transform API similar to browserify's: each file goes through a series of transforms streams, the output of one becoming the input of the next. Recently an increasing number of transform streams have appeared that operate on plain old parseable JavaScript, generally using the following pattern:

var escodegen = require('escodegen')
var through = require('through')
var esprima = require('esprima')
var modify = require('./modify')

module.exports = function(file, opts) {
  var buffer = []

  return through(function(data) {
    buffer.push(data)
  }, function() {
    buffer = buffer.join('\n')

    var ast = esprima.parse(buffer)

    // Whatever functionality goes here...
    modify(ast)

    buffer = escodegen.generate(ast)
    this.queue(buffer)
    this.queue(null)
  })
}

This is straightforward and pretty repeatable, but if you start chaining together multiple transforms together that work like this, you get increasing overhead from parsing/deparsing the JavaScript AST. In larger projects, this can make builds take considerably longer than they would without the transforms.

So, ast-pipeline lets you pass in an array of streams as normal, but also allows you to pass in functions too. These functions are expected to take an AST and transform it, calling a callback when they're ready to pass it on to the next transform. If you chain a few AST transforms together, you'll only have to parse/deparse once!

Switching between text transform streams and AST modifiers is handled transparently, so you can still pass transforms in any order and they'll be converted to the right format as required.

Usage

createPipeline = astPipeline([opts])

Creates a new pipeline "factory". Takes the following options:

• opts.preferAST: expect a single AST object as both the input and ouptut of these pipelines.

createPipeline.decode(handle)

Specify how to convert from a buffered text stream to an AST:

var css = require('css')

pipeline.decode(function(contents, next) {
  var ast = css.parse(contents)
  next(null, ast)
})

createPipeline.encode(handle)

Specify how to convert from an AST object to a transform stream:

var css = require('css')

pipeline.encode(function(ast, next) {
  var contents = css.stringify(ast)
  next(null, contents)
})

createPipeline(transforms)

Takes an array of Stream and Function transforms, returning a duplex stream which will take the inital file input and output the transformed contents.

var pipeline = require('ast-pipeline')
var css = require('css')
var fs = require('fs')
var bl = require('bl')

function reworker() {
  return pipeline().encode(function(ast, done) {
    done(null, css.stringify(ast))
  }).decode(function(ast, done) {
    done(null, css.parse(ast))
  })
}

function transformFile(filename, transforms, callback) {
  transforms = transforms.map(function(tr) {
    return tr(filename)
  })

  fs.createReadStream(filename)
    .pipe(reworker()(transforms))
    .pipe(bl(callback))
}

AST Transforms

AST transforms are simply functions with the signature (ast, done). They take an object (generally, an AST), modify it, and pass it back to done(err, ast).

If no new value is passed through the done callback, the previous value will be used instead.

var variables = require('rework-variables')

module.exports = function(file) {
  return function(ast, done) {
    variables({
      hello: 'world'
    })(ast.stylesheet)

    done(null, ast)
  }
}

License

MIT. See LICENSE.md for details.