Define constants to replace with expression

This plugin allows replace identifier with other expression or literal, for example when we want to specify in webpack's config different parameters for our code.

Consider the following example, in which we would like to use different code dependant on the parameter passed from webpack/.babelrc:

import LiveUI from './LiveUI';
import ReportViewer from './ReportViewer';

export { __ACTIVE_CONFIG__ as default };

Config example:

.babelrc

{
   "presets": ["es2015", "react", "stage-1"],
   "plugins": [
     ["transform-decorators-legacy"],
     ["transform-inline-environment-variables"],
     [ "define", {
        "__API_ROOT__": "document.location.origin + '/api'",
        "__SOCKET_ROOT__": "document.location.origin",
        "__DEBUG__": "false",
        "__ACTIVE_CONFIG__": "LiveUI"
     } ]

   ]
}

webpack.config.js

var defines = {
  '__API_ROOT__': 'document.location.origin + "/api"',
  '__SOCKET_ROOT__': 'document.location.origin',
  '__DEBUG__': JSON.stringify( JSON.parse( process.env.BUILD_DEBUG || 'false' ) ),
  '__ACTIVE_CONFIG__': 'LiveUI'
};

var babelConfig = JSON.parse( fs.readFileSync('.babelrc') );
babelConfig.plugins.push( [ "define", defines ] );

...
module.exports = {
  ...
  module: {
    loaders: [
      ...
      { test: /\.js?$/, loader: 'babel', exclude: /node_modules|bower_components/, query: babelConfig },
      ...
    ]
  }
};