context-map-webpack-plugin

Webpack plugin to map dynamic (thirdparty) dependencies to a static context map.

Install

npm install -D context-map-webpack-plugin

Why do I need it / What does it do?

The Problem

Some (thirdparty) code requires other files with expressions that cannot be resolved by webpack at compile-time. Webpack tries to generate a regular expression that determines which files to include in the webpack bundle. In more complex expressions, webpack is unable to determine which files to include, thus an empty context map will be created.

A dynamic import that cannot be resolved might look like this:

// ...
var FILES = UglifyJS.FILES = [
    "../lib/utils.js",
    "../lib/ast.js",
    "../lib/parse.js",
    "../lib/transform.js",
    "../lib/scope.js",
    "../lib/output.js",
    "../lib/compress.js",
    "../lib/sourcemap.js",
    "../lib/mozilla-ast.js",
    "../lib/propmangle.js",
    "./exports.js",
].map(function(file){
    return require.resolve(file);
});
// ...

Your webpack build will probably have this Critical dependency warning:

WARNING in ./node_modules/uglify-js/tools/node.js 24:11-32
Critical dependency: the request of a dependency is an expression
 @ ./node_modules/pug-filters/lib/run-filter.js
 @ ./node_modules/pug-filters/index.js
 @ ./node_modules/pug/lib/index.js
 @ ./src/server/PugHandler.ts
 @ ./src/server/Router.ts
 @ ./src/server/index.ts
 @ multi ./src/server/index.ts webpack/hot/poll?1000

This will result in a bundle output that looks like this:

// ...
***/ "./node_modules/uglify-js/tools sync recursive":
/*!*******************************************!*\
  !*** ./node_modules/uglify-js/tools sync ***!
  \*******************************************/
/*! no static exports found */
/***/ (function(module, exports) {

eval("function webpackEmptyContext(req) {\n\tvar e = new Error(\"Cannot find module '\" + req + \"'\");\n\te.code = 'MODULE_NOT_FOUND';\n\tthrow e;\n}
// ...

This empty context will lead to runtime errors:

node ./dist/server/main.js
undefined:4
	throw e;
	^

Error: Cannot find module '../lib/utils.js'
    at Function.webpackEmptyContext [as resolve] (eval at ./node_modules/uglify-js/tools sync recursive (<project_dir>/dist/server/main.js:5371:1), <anonymous>:2:10)
// ...

The Solution

Create a static context map in compile-time in order to include all required dependencies. You are required to investigate all possible runtime values of the required argument where the error originates from. These values have to be passed as the staticImports argument in the plugin constructor.

Alternatives

If you use webpack for the backend, you can exclude the required files from the webpack bundle and depend on node_mdules by using webpack-node-externals.

Usage

In your webpack configuration:

const ContextMapPlugin = require('context-map-webpack-plugin');

module.exports = {
  // ...
  plugins: [
    new ContextMapPlugin(contextPath, staticImports)
  ],
  // ...
};

Parameters

contextPath

• Type: String
• Default: undefined

Specify the directory in which to create/overwrite the context map.

staticImports

• Type: Array[String]
• Default: undefined

Static imports that will be written to the context map (and bundled with webpack at compile-time).

Example (uglify-js)

  // ...
  plugins: [
    new ContextMapPlugin('node_modules/uglify-js/tools', [
      '../lib/utils.js',
      '../lib/ast.js',
      '../lib/parse.js',
      '../lib/transform.js',
      '../lib/scope.js',
      '../lib/output.js',
      '../lib/compress.js',
      '../lib/sourcemap.js',
      '../lib/mozilla-ast.js',
      '../lib/propmangle.js',
      './exports.js',
    ])
  ],
  // ...

License

MIT