underscore chunk method

chunk.js

// Turns a list into a list of lists of specified lengths.
_.chunk = function(array,chunkSize) {
	return _.reduce(array,function(reducer,item,index) {
		reducer.current.push(item);
		if(reducer.current.length === chunkSize || index + 1 === array.length) {
			reducer.chunks.push(reducer.current);
			reducer.current = [];
		}
		return reducer;
	},{current:[],chunks: []}).chunks
};

// test

"chunk reduces an array to chunks": function() {
	var chunked = _.chunk(["a","a","b","b","c"],2);
	assert.equals(chunked[0],["a","a"]);
	assert.equals(chunked[2],["c"]);
}

Oh hai @timruffles. I just found myself doing the same thing for edutainment, and then stumbled on this here beauty!

We ended up with:

// i split an array into an array of arrays of size `chunk`
function chunk (array, size) {
  return array.reduce(function (res, item, index) {
    if (index % size === 0) { res.push([]); }
    res[res.length-1].push(item);
    return res;
  }, []);
}

...for other internet travellers who stumble here, you'll probably want something like: https://lodash.com/docs#chunk