Webpack in-memory filesystem

inmemory-webpack-compiler.js

var webpack = require('webpack');
var MemoryFS = require('memory-fs');
var SingleEntryDependency = require('webpack/lib/dependencies/SingleEntryDependency');
var fs = new MemoryFS();

fs.mkdirpSync('/src');
fs.writeFileSync('/src/app.js', 'require("./dep.js")', 'utf-8');
fs.writeFileSync('/src/dep.js', 'module.exports = function(msg){console.log(msg)}', 'utf-8');
fs.writeFileSync('/src/extra-entry.js', 'require("./dep.js")', 'utf-8');

var compiler = webpack({
  context: '/src',
  entry: {app: './app.js'},
  output: {
    filename: '[name].js',
    path: '/build'
  }
});

compiler.inputFileSystem = fs;
compiler.outputFileSystem = fs;
compiler.resolvers.normal.fileSystem = fs;
compiler.resolvers.context.fileSystem = fs;

compiler.plugin('make', function(compilation, done) {
  var dep = new SingleEntryDependency('./extra-entry.js');
  dep.loc = 'extra-entry';

  compilation.addEntry('/src', dep, 'extra-entry', done);
});

compiler.run(function(err, stats) {
  if (err) throw err;

  console.log(stats.compilation.assets['app.js'].source());
});

Thanks! Great Gist!

As a side note. Webpack 5 uses compiler.intermediateFileSystem = fs; instead of compiler.resolvers.normal and compiler.resolvers.context.