code: function mapModule(state, module) {
    var moduleMap = state.opts.map || {};
    if (moduleMap.hasOwnProperty(module)) {
      return moduleMap[module];
    }
    // Jest understands the haste module system, so leave modules intact.
    if (process.env.NODE_ENV !== 'test') {
      var modulePrefix = state.opts.prefix;
      if (modulePrefix == null) {
        modulePrefix = './';
      }
      return modulePrefix + module;
    }
    return null;
  }
docs: Rewrites module string literals according to the `map` and `prefix` options.
  This allows other npm packages to be published and used directly without
  being a part of the same build.