Webpack Recompilation Simulator

If you are writting a webpack loader or a webpack plugin you might want to test the efficiency of your caching solutions.

The Webpack Recompilation Simulator allows you to run the webpack compilation multiple times and also to simulate file changes on the real file system just like a user would do.

How does it work?

The simulator hooks into webpack using the beforeResolve hook of the normalModuleFactory.
Inside the beforeResolve hook it will map all files you want to modify during your tests to a temporary file on the disk.

The temporary files are stored using temp-fs and can be altered using the simulateFileChange method provided by the Webpack Recompilation Simulator.

Api

Creating a new instance

To create an instance of the Webpack Recompilation Simulator you have to pass a webpack compiler:

const compiler = webpack(config);
const webpackSimulator = new WebpackRecompilationSimulator(compiler);

Add all files you might want to modify during your test

To make sure that webpack will add the correct file dependencies we have to add the test files before we run any tests:

webpackSimulator.addTestFile(__dirname + '/src/main.js');

Start a compilation

The webpack simulator will start a new compiler run:

webpackSimulator
  .run()  
  .then(function(stats) {
    console.log(stats);
  });

Simulate a file change

You can prepend a banner, append a footer or replace the entire content of the file.
Make sure that you added it using addTestFile.

    webpackSimulator.simulateFileChange(testFilePath, {
      banner: '/* Add a header */'
    });

    webpackSimulator.simulateFileChange(testFilePath, {
      content: '/* Overwrite the entire file */'
    });

    webpackSimulator.simulateFileChange(testFilePath, {
      footer: '/* Add a footer */'
    });

Simulate a file change using the node api

It is also possible to directly modify files using the fs api

``js const tempMainFile = webpackSimulator.addTestFile(__dirname + '/src/main.js');

fs.writeFileSync(tempMainFile, '// Hello world'); ``

Examples

Watch mode

  const compiler = webpack(config);
  const webpackSimulator = new WebpackRecompilationHelper(compiler);
  
  const tempMainFile = webpackSimulator.addTestFile(__dirname + '/src/main.js');
 
  // Start watching (this will also trigger an initial build)
  webpackSimulator.startWatching()  
    .then((stats) => {
      console.log('Initial compilation result:', stats);
      // Simulate a change to the main.js file 
      fs.writeFileSync(tempMainFile, 'require("./demo.js");');
      // Wait until webpack detects the file change and compiles again
      return webpackSimulator.waitForWatchRunComplete();
    })
    .then((stats) => {
      console.log('Partial compilation result:', stats);
    })
    // Stop watching
    .then(() => {
      return webpackSimulator.stopWatching();
    });

Direct compile runs

  const compiler = webpack(config);
  const webpackSimulator = new WebpackRecompilationHelper(compiler);
  
  const tempMainFile = webpackSimulator.addTestFile(__dirname + '/src/main.js');
 
  webpackSimulator.run()  
    .then(function(stats) {
      console.log('Initial compilation result:', stats);
      // Simulate a change to the main.js file
      fs.writeFileSync(tempMainFile, 'require("./demo.js");');
      // Compile it again:
      return webpackSimulator.run();
    })
    .then(function(stats) {
      console.log('Partial compilation result:', stats);
    });

Options

• tmpDirectory: In order to simulate file changes this util needs a temporary directory. Default: process.cwd() + '/tmp/'

Motiviation

The Webpack Recompilation Simulator is used to test the https://github.com/jantimon/html-webpack-plugin

Requirements

• Webpack 4
• Node 6+

Known Issues

• Webpack will only cache loaders in development mode