starts and ends with mixins

startsOrEndsWithMixins.js

(function() {
  'use strict';
  _.mixin({
    startsWith: function startsWithMixin(s1, s2) {
      return startsOrEndsWith(_.first, s1, s2);
    },
    endsWith: function endsWithMixin(s1, s2) {
      return startsOrEndsWith(_.last, s1, s2);
    }
  });
  function startsOrEndsWith(fn, s1, s2) {
    return s1 && s2 && s1.length > s2.length && fn(s1, s2.length).join('') === s2;
  }
})();