// Поддержка на старых браузерах 
 if (!String.prototype.endsWith) { 
     Object.defineProperty(String.prototype, 'endsWith', { 
         value: function(searchString, position) { 
             var subjectString = this.toString(); 
             if (position === undefined || position > subjectString.length) { 
                 position = subjectString.length; 
             } 
             position -= searchString.length; 
             var lastIndex = subjectString.indexOf(searchString, position); 
             return lastIndex !== -1 && lastIndex === position; 
         } 
     }); 
 } 
  
 var words = ['test', 'testing', 'the test']; 
 var words_includes = []; 
 for (var index = 0; index <= words.length - 2; index++) { 
     if (words[index + 1].endsWith(words[index].slice(0, 1))) { 
         words_includes.push(words[ index ]); 
     } 
 } 
  
 console.log(words_includes);