This is functional JavaScript - composition first

filter-etcd-config.js

require('cc-globals'); //establishes fetch and R (ramda library) globally

//regular expression for valid etcd address
var address = /^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\:\d{1,5}$/;

//if the input is a string, makes it an array containing the string, otherwise returns the original input
var fixStr = (v) => typeof v === 'string' ? [v] : v;

//converts collection input to an array, returns empty array otherwise
var fixArray = (v) => v && v.length && Array.prototype.slice.call(v) || [];

//forces input to string and trims it
var mapFixString = R.map((v) => (v || '').toString().trim());

//function will act as filter against an array, each item must match the address regex
var validFilter = R.filter((v) => address.test(v));

//will filter out an empty array, returning null, or an array with items
var checkList = (a) => a.length ? a : null;

//method wrapping composition of filters
var filterEtcdConfig = R.compose(checkList, validFilter, mapFixString, fixArray, fixStr);

//expose final composed method
module.exports = filterEtcdConfig;

filter-etcd-config.tests.js

require('cc-globals');

//utilities for testing
var chai = require('chai')
  ,expect = chai.expect
  ;

describe('filter-etcd-config', function(){
  var sut = require('../../../src/normalize-options/filter-etcd-config');

  it('will return expected valid values', function(){
    expect(sut(['127.1.0.1:4001','invalid',null,123])).to.deep.equal(['127.1.0.1:4001']);
  });

  it('will convert a string', function(){
    expect(sut('127.1.0.2:4001')).to.deep.equal(['127.1.0.2:4001']);
  });

  it('will return null for invalid or empty results',function(){
    return expect(sut(-1)).to.be.null;
  });

});