Question

I am using node.js to build a tcp server, and I want to extract integers from the data received.

var net = require('net');
var server = net.createServer(function (socket) {
  socket.setEncoding('ascii');
  socket.addListener("data", function (data) {
    var pkgDataContent = data.substr(0, 2);
  });
});
server.listen(1337, "192.168.80.91");

The data received is string type, and the numbers are 1 byte, 2 bytes and 4 bytes. How to extract these 1-byte, 2-byte and 4-byte integers from a javascript string? Like the code above: pkgDataContent is a string of 2 bytes, but actually it is an integer, how to convert it to javascript number correctly?

Answer 1

Depends on endianness and on whether it’s signed or not.

big endian 32-bit unsigned integer:

pkgDataContent.charCodeAt(0) << (8*3) +
pkgDataContent.charCodeAt(1) << (8*2) +
pkgDataContent.charCodeAt(2) << (8*1) +
pkgDataContent.charCodeAt(3) << (8*0)

little endian 32-bit unsigned integer:

pkgDataContent.charCodeAt(3) << (8*0) +
pkgDataContent.charCodeAt(2) << (8*1) +
pkgDataContent.charCodeAt(1) << (8*2) +
pkgDataContent.charCodeAt(0) << (8*3)