Thursday, January 8, 2009

MapReduce with JavaScript

Inspired by Michael Nielsen's Write your first MapReduce program in 20 minutes, I've ported his example to JavaScript.

The m/r kernel is pretty small, actually just a single mapReduce function and a helper groupBy function:

// mapper should return an array of [{key:'somekey', value:'somevalue'}]
// reducer should return a single {key:'somekey', value:'somevalue'}
function mapReduce(i, mapper, reducer) {
 var intermediate = [];
 var output = [];
 for(var key in i) {
  var value = i[key];
  intermediate = intermediate.concat(mapper(key, value));

 var groups = groupBy(intermediate);
 for(var key in groups) {
  var values = groups[key];
  output.push(reducer(key, values));

 return output;

// list should be [{key:k, value:v}, ....] where key may be repeated.
// returns [{key, [v1, v2, v3...]}, ...] where key is *not* repeated.
function groupBy(list) {
 var ret = {};
 for (var i=0; i<list.length; i++) {
  var key = list[i].key;
  var value = list[i].value;
  if (!ret[key]) {
   ret[key] = [];

 return ret;

And you could use it for the canonical "word count" example like so:

function myMapper(key, value) {
 var ret = [];
 var words = normalizeText(value).split(' ');
 for (var i=0; i<words.length; i++) {
  ret.push({key:words[i], value:1});
 return ret;

function myReducer(intermediateKey, values) {
 var sum = 0;
 for (var i=0; i<values.length; i++) {
  sum += values[i];
 return {key:intermediateKey, value:sum};

function normalizeText(s) {
 s = s.toLowerCase();
 s = s.replace(/[^a-z]+/g, ' ');
 return s;

var i = {};
i.atxt = "The quick brown fox jumped over the lazy grey dogs.";
i.btxt = "That's one small step for a man, one giant leap for mankind.";
i.ctxt = "Mary had a little lamb, Its fleece was white as snow; And everywhere that Mary went, The lamb was sure to go.";

var out = mapReduce(i, myMapper, myReducer);

This example just allows you to write programs in the MapReduce style. It doesn't do any of the fancy footwork necessary to actually parallelize the operations and manage the execution.

That does make me think of some interesting possibilities though.

If only there was a vast sea of computers, all running javascript interpreters, all connected to the internet, all capable of downloading and running your m/r jobs. :)


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  4. Nice little script.

    I added a small "improvement" to support array inputs and parameterized keys.

    Makes it more useful for feeding client charting libraries like flot or jqPlot.

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