An implementation of a solution to a "kata" at http://www.codewars.com/dojo/katas/5245a9138ca049e9a10007b8/train/javascript

count-adjacent-appearances-of-same-word-in-text.js

function countAdjacentPairs(search_string) {
	var words_that_appeared_in_adjacent_pairs = [];

	sets_of_two_adjacent_words_in( search_string ).forEach( function( two_adjacent_words ) {
		var first_of_the_two_words = two_adjacent_words[ 0 ];
		if (
			two_adjacent_words.are_a_pair()
			&& first_of_the_two_words.is_not_one_of( words_that_appeared_in_adjacent_pairs )
		) {
			// which one reads more naturally?
			//words_that_appeared_in_adjacent_pairs.add_word( first_of_the_two_words );
			add_to( words_that_appeared_in_adjacent_pairs, first_of_the_two_words );
		}
	} );

	return words_that_appeared_in_adjacent_pairs.length;
}

nitty-gritty.js

"use strict";

Array.prototype.are_a_pair = function () {
	var first_word_in_lowercase = this[ 0 ].toLowerCase(),
		second_word_in_lowercase = this[ 1 ].toLowerCase();

	return first_word_in_lowercase === second_word_in_lowercase;
};

add_to = function( collection, word ) {
	collection.push( word.toLowerCase() );
};

Array.prototype.contains = function( element_to_check_for ) {
	return -1 !== this.indexOf( element_to_check_for.toLowerCase() );
};

String.prototype.is_one_of = function( collection_to_check_in ) {
	return collection_to_check_in.contains( this );
};

String.prototype.is_not_one_of = function( collection_to_check_in ) {
  return ! this.is_one_of( collection_to_check_in );
};

function sets_of_two_adjacent_words_in( search_string ) {
	var words_in_search_string = search_string.split( ' ' ),
		sets = [],
		num_words_in_search_string = words_in_search_string.length;

	if ( num_words_in_search_string > 1 ) {
		for( var i = 0; i < num_words_in_search_string - 1; i++ ) {
			sets.push( [ words_in_search_string[ i ], words_in_search_string[ i+1 ] ] );
		}
	}

	return sets;
}

tests.js

Test.assertEquals( [ 'cat', 'dog' ].are_a_pair(), false, "'cat' and 'dog' are not a pair" );
Test.assertEquals( [ 'dog', 'dog' ].are_a_pair(), true, "'dog' and 'dog' are a pair" );
Test.assertEquals( [ 'dog', 'dOg' ].are_a_pair(), true, "'dog' and 'dOg' are a pair" );

Test.assertEquals( 'dog'.is_not_one_of( [ 'cat' ] ), true, "'dog' is not one of [ 'cat' ]" );
Test.assertEquals( 'dog'.is_one_of( [ 'dog' ] ), true, "'dog' is one of [ 'dog' ]" );

Test.assertEquals( ['dog'].contains( 'dog' ), true, "[ 'dog' ] contains 'dog'" );
Test.assertEquals( ['dog'].contains( 'cat' ), false, "[ 'dog' ] does not contain 'cat'" );

Test.assertEquals( countAdjacentPairs('cat cat dog dog bird bird'), 3, 'countAdjacentPairs() counts adjacent pairs in provided string' );
Test.assertEquals( countAdjacentPairs(''), 0, 'An empty string contains 0 adjacent pairs' );
Test.assertEquals( countAdjacentPairs('dog cat dog'), 0, 'Only adjacent pairs are counted' );
Test.assertEquals( countAdjacentPairs('dog dog dog'), 1, 'Pairs that repeat are only counted once' );
Test.assertEquals( countAdjacentPairs('cat CaT dog dOg'), 2, 'Words are pairs even if the case of the letters is different' );