index.js
function next () {
    var args = slice.call(arguments)
    if (!isYieldable) {
      // only handle callback if not yieldable
      if (iter && yielded) {
        // no need defer when yielded
        step.apply(self, args)
      } else {
        // need next tick if not defered
        process.nextTick(function () {
          step.apply(self, args)
        })
      }
    } else {
      step(new Error('Callback on yieldable is prohibited'))
    }
  }
function next () {
    var args = slice.call(arguments)
    if (!isYieldable) {
      // only handle callback if not yieldable
      if (iter && yielded) {
        // no need defer when yielded
        step.apply(self, args)
      } else {
        // need next tick if not defered
        process.nextTick(function () {
          step.apply(self, args)
        })
      }
    } else {
      step(new Error('Callback on yieldable is prohibited'))
    }
  }
function next ( ) { var args = slice . call ( arguments ) if ( ! isYieldable ) { // only handle callback if not yieldable if ( iter && yielded ) { // no need defer when yielded step . apply ( self , args ) } else { // need next tick if not defered process . nextTick ( function ( ) { step . apply ( self , args ) } ) } } else { step ( new Error ( 'Callback on yieldable is prohibited' ) ) } }
next, callback stepper with nextTick

@param {object} err - callback error object
@param {...*} val - callback value(s)
next callback stepper with nextTick