Hashcash is a simple proof of work algorithm to prevent or slow down spam. It works by giving the requester a challenge in the form of a sha1 hash. After the hash is received the requesting client has four seconds to find a hash of the challenge + number that has three leading zeros. The client can find the challenge in the header: hashcash-challenge and add the answer to hashcash-answer.

Using the middleware on an end-point in express will prevent the client from accessing the resource until an answer is found. A new challenge is created every four seconds.

npm install hashcash 

First, require hashcash:

var hashcash = require('hashcash');
var express = require('express');
var app = express.createServer();

app.get('/protected',hashcash.middleware(),function(req,res) {
	/* this will never get hit if the hashcash-answer is not present */
	res.send('Success!');
});

var port = 8080;
console.log('Listening on port: '+port);
app.listen(port);