SQL-like left joins in JavaScript

left_join.js

var _  = require('underscore');

var geo_key   = 'name',
		value_key = 'state_name';

var geo_data = [ 
		{
			"name": 'AK',
			"geom": '1'
		}, 
		{
			"name": 'CA',
			"geom": '2'
		}, 
		{
			"name": 'NY',
			"geom": '3'
		},
		{
			"name": 'LA',
			"geom": '4'
		}
	]

var value_data = [ 
		{
			"state_name": 'AK',
			"awesomeness": '1',
			"coldness": 200
		}, 
		{
			"state_name": 'CA',
			"awesomeness": '100'
		}, 
		{
			"state_name": 'NY',
			"awesomeness": '75'
		}
	]

function addNulls(data, null_key_obj){
	data.forEach(function(datum){
		// Create copies of our objects so they dont' get overwritten
		var null_key_obj_persist = _.extend({}, null_key_obj),
				datum_persist        = _.extend({}, datum);
		// You could extend `null_key_obj_persist` with `datum` but that would reverse the order of your keys
		// And always put your keys that have nulls (which are probably the least important keys) first.
		// This way will overwrite everything with nulls, then rewrite keys that have values.
		_.extend(datum, null_key_obj_persist, datum_persist)
	})
	return data
}

function addToNullMatch(key_map, keys){
	keys.forEach(function(key){
		if (!key_map.null_match[key]){
			key_map.null_match[key] = null
		}
	})
}

function indexRightDataOnKey(right_data, right_key_column){
	var key_map = {
			null_match: {}
	};

	right_data.forEach(function(datum){
		var right_key_value = datum[right_key_column];
		if (!key_map[right_key_value]){
			// Get rid of the original name key since that will just be a duplicate
			delete datum[right_key_column];
			key_map[right_key_value] = datum;
			// Log the new keys that we've encountered for a comprehensive list at the end
			addToNullMatch(key_map, _.keys(datum));
		}else{
			throw new Error('Duplicate entry for "' +  right_key_value + '"');
		}
	})
	return key_map
}

function joinOnMatch(left_data, left_key_column, key_map){
	left_data.forEach(function(datum){
		var key   = datum[left_key_column],
		    match = key_map[key];
		if (match){
			_.extend(datum, match);
		}
	})
	return left_data
}

function joinData(left_data, left_key_column, right_data, right_key_column){
	var key_map             = indexRightDataOnKey(right_data, right_key_column),
			joined_data         = joinOnMatch(left_data, left_key_column, key_map),
			joined_data_w_nulls = addNulls(joined_data, key_map.null_match);
	return joined_data_w_nulls;
}

var joined_data = joinData(geo_data, geo_key, value_data, value_key)


console.log(joined_data)
/*
[ { name: 'AK', geom: '1', awesomeness: '1', coldness: 200 },
  { name: 'CA', geom: '2', awesomeness: '100', coldness: null },
  { name: 'NY', geom: '3', awesomeness: '75', coldness: null },
  { name: 'LA', geom: '4', awesomeness: null, coldness: null } ]
*/