index.js

module.exports = function(content, map, meta) {
  // we skip at the build and the runtime level, to not include this loader in
  // production mode, regardless of how the developer is defining "production mode"
  if (process.env.NODE_ENV === "production") {
    this.callback(null, content, map, meta);
    return;
  }

  var newContent = content.replace(/module.exports = /, "var rawLocals = ");
  newContent += `

  // IE 11 etc. cannot use Proxy
  var canUseProxy = typeof Proxy !== 'undefined' && Proxy.constructor;

  // we skip at the build and the runtime level, to not include this loader in
  // production mode, regardless of how the developer is defining "production mode"
  var shouldUseProxy = process.env.NODE_ENV !== 'production';

  var resourcePath = module.i;

  var resourceParents = module.parents.map(function(parent) {
    return '- ' + parent;
  }).join('\\n');

  module.exports = canUseProxy && shouldUseProxy
    ? new Proxy(rawLocals, {
      get: function(obj, prop) {
        if(obj.hasOwnProperty(prop) || prop === '__esModule') { return obj[prop]; }
        else {
          var allowedKeys = Object.keys(obj);
          var temp = allowedKeys.map(function(k) { return '- ' + k; }).join('\\n');
          console.error('[strict-css-modules-loader]\\n.' + prop.toString() + '\\nThe CSS class "' + prop.toString() + '" does not exist in:\\n'+ resourcePath + '\\nUsed by:\\n' + resourceParents + '\\nThe available classes are: \\n' + temp);
        }
      },
      set: function(obj, prop, value) {
        console.error(
          '[strict-css-modules-loader]There should be no good reason to set the styles of ' + resourcePath + ' dynamically! ' + prop + ' is being set to ' + value);
        obj[prop] = value;
      }
    })
  : rawLocals`;

  this.callback(null, newContent, map, meta);
};