Converting the WordArray into 8-bit character values.

to_buffer.js

function to_char(number){
	return [
		(number & 0xff000000) >>> 24,
		(number & 0x00ff0000) >>> 16,
		(number & 0x0000ff00) >>> 8,
		(number & 0x000000ff) >>> 0
	];
}

function to_char_array(wordarray){
	result = [];
	for (var index in wordarray.words) {
		word = wordarray.words[index];
		result = result.concat(to_char(word));
	}
	return result;
}

// bonus: populating Node.JS's Buffer type
function to_buffer(wordarray) {
	words = wordarray.words;
	buffer = Buffer(wordarray.sigBytes);

	for (var x = 0; x < words.length; x++) {
		word = to_char(words[x]);
		for (var y = 0; y < 4; y ++){
			buffer[x*4+y] = word[y];
		}
	}
	return buffer;
}

// test
var wordarray = {
	sigBytes: 16,
	"words": [
		3150126731,
		5845652552,
		7032008983,
		1211931708
	]
};

console.log(to_buffer(wordarray));
console.log(to_char_array(wordarray));