Adds the contents of a virtual file to webpack's cached file system without writing it to disk
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examples v0.4.0 release Mar 5, 2018
index.js Fix deprecation warning in webpack 4 Dec 12, 2018

Virtual Module Webpack Plugin Build Status npm package

This is an experimental plugin that adds the contents of a virtual file to Webpack's cached file system without writing it to disk.

This would be used if you generated file contents at build time that needs to be consumed as a module by your source code, but you don't want to write this file to disk.

It uses private APIs of the CachedInputFileSystem of the enhanced-resolve package that Webpack uses as the module resolver. Therefore, it is inherently fragile and subject to be broken if the CachedInputFileSystem changes. Fortunately, the changes have not been too extensive between webpack 1.x - 4.x and this plugin has been updated to be compatible with all.


If another webpack plugin clears the CachedInputFileSystem without triggering the resolve event of the resolver plugin lifecycle, the virtual file will no longer be able to be referenced. Based off the issues received in this plugin's history, this does not seem to be an issue.

Difference between val-loader

val-loader is also capable of dynamically generating module code at build time. val-loader is a "loader" and not a "plugin." Webpack loaders require a file to exist in webpack's file system cache. Webpack loads the cache from the files on disk. This virtual-module-webpack-plugin inserts directly into webpack's file system cache.

val-loader is better if you have a file you want to load at build time that contains all of the logic to dynamically fetch and return the source of that file. You also are able to use watch mode in development since there is a physical file to watch.

virtual-module-webpack-plugin is better if you have a build script that is collecting stats, config or other data that you want to be able to reference in the runtime code without every writing that data to a source file on disk.


In your webpack.config.js, require the plugin:

const VirtualModulePlugin = require('./virtual-module-webpack-plugin');

Then when defining the config object create an instance of the plugin passing in the moduleName and contents and add it to the webpack config's plugins array.

The moduleName should be relative to your webpack config context which defaults to the directory holding the webpack.config.js file.

  plugins: [
    new VirtualModulePlugin({
      moduleName: 'src/mysettings.json',
      contents: JSON.stringify({ greeting: 'Hello!' })

Then require the file as you would any other module in your source. The file contents will be passed through any loaders you setup that match the moduleName.

If you pass an object to contents, it will automatically be passed through JSON.stringify. You can also pass a function to contents which will be invoked at compile time with no arguments. See pull #10.

See the examples directory for a complete working examples with webpack 1.x, 2.x and 4.x.

If you need to fetch the contents asynchronously, you need to have your webpack.config.js return a Promise. It should first resolve getting your module contents and then return the Webpack config.

A few development attempts were made at letting the plugin resolve the contents on demand, but we were unable to get Webpack to wait for a callback during the resolve stage. See pull requests #11 and #12. Pull requests to solve the problem are welcome, but it needs to work even if the asynchronous content request takes a while. You can uncomment code in test/integration/cases/contents-async/webpack.config.js to test it.

Here is an example of async content fetching inside webpack.config.js:

'use strict';

const VirtualModulePlugin = require('virtual-module-webpack-plugin');

function contents() {
  return new Promise(resolve => {
    setTimeout(() => {
    }, 1000);

module.exports = contents().then(asyncContents => ({
  entry: './index',
  plugins: [
    new VirtualModulePlugin({
      moduleName: './a.txt',
      contents: asyncContents,