a.feature

Feature: A

  Scenario: a
    Given a

a.js

module.exports = function () {
  this.Before(function (scenario, callback) {
    console.log("world:", this, "scenario:", scenario, "callback:", callback);
    callback();
  });
};

gistfile1.txt

‣ cucumber.js a.feature 
world: {} scenario: { payloadType: 'scenario',
  setBackground: [Function: setBackground],
  buildScenarios: [Function: buildScenarios],
  getKeyword: [Function: getKeyword],
  getName: [Function: getName],
  getDescription: [Function: getDescription],
  getUri: [Function: getUri],
  getLine: [Function: getLine],
  getBackground: [Function: getBackground],
  addStep: [Function: addStep],
  getLastStep: [Function: getLastStep],
  getMaxStepLength: [Function],
  setSteps: [Function: setSteps],
  getSteps: [Function: getSteps],
  addTags: [Function: addTags],
  addInheritedtags: [Function: addInheritedtags],
  getTags: [Function: getTags],
  getOwnTags: [Function: getOwnTags],
  acceptVisitor: [Function: acceptVisitor],
  instructVisitorToVisitBackgroundSteps: [Function: instructVisitorToVisitBackgroundSteps],
  instructVisitorToVisitScenarioSteps: [Function: instructVisitorToVisitScenarioSteps],
  instructVisitorToVisitSteps: [Function: instructVisitorToVisitSteps] } callback: function () {
          iterate();
        }
U

1 scenario (1 undefined)
1 step (1 undefined)

You can implement step definitions for undefined steps with these snippets:

this.Given(/^a$/, function (callback) {
  // Write code here that turns the phrase above into concrete actions
  callback.pending();
});