A quick little script showing someone a general outline of bridging a hardware SPI device to an HTTP api via node.js. It's fairly terse, but I tried to provide a few documentation and project links for further exploration.

spi_http_example.js

var SPI_DEV = "/dev/spidev0.0",     // hardware will be connected to pins matching a certain Linux SPI device
    HTTP_PORT = 0,          // if you leave 0, a port gets randomly assigned
    POLL_INTERVAL = 1e3;    // equivalent to 1 second, set to `20` for 20 milliseconds

// this global variable will store the values 
var most_recent_value = null;


var spiInstance = require('pi-spi').initialize(SPI_DEV);      // this sets up the SPI device for use in node.js
// then we register a timeout that will get called repeatedly at an approximate interval
setInterval(function () { // this function gets called every POLL_INTERVAL milliseconds

  // inside the timeout callback function, we call the SPI "instance" read method to sipmly read a 4-byte response
  // this here is where you might need to do a "spiInstance.transfer" instead and write a command or something so the device knows what you want it to do.
  
  spiInstance.read(4, function (e,d) {      // this function is a "callback", and will get run when the read completes
    // `e` will contain an Error object if something went wrong
    // otherwise on success, `e` will be null, and `d` a buffer
    if (e) console.error(e);
    else most_recent_value = d.readFloatLE(0);     // see https://nodejs.org/api/buffer.html#buffer_buf_readfloatle_offset_noassert
    
    // for more complicated data structures (here we assume the SPI device just sends a single 4-byte float number whenever it is talked to) you might look into https://github.com/natevw/struct-fu or similar.
    // you might also want to encapsulate the device logic into a separate module to keep the code clean and/or allow others to reuse it. see https://github.com/natevw/node-nrf for a fairly complicated example of that.
    
  });
  
}, POLL_INTERVAL);


// this is a really barebones HTTP server that just "barfs out" the `most_recent_value` for anyone to see
// see https://nodejs.org/api/http.html for node.js server method documentation
require('http').createServer(function (req, res) {
  
  // normally you might look at `req` to see what method/path/etc. it has
  // (for significant amounts of HTTP processing you might use the Express library to help)
  
  // in our case we just respond to any sort of request with a simple response.
  res.writeHead(200, {'Content-Type': "text/plain"});
  res.write("Most recent value is: ")
  res.end( most_recent_value.toFixed(2) );      // this writes a bit more data and then finishes
  
}).listen(HTTP_PORT, function () {
  console.log("HTTP server listening on", this.address().port);
}).on('error', function (e) {
  console.error(e);
});