How to calculate the current CPU load with Node.js; without using any external modules or OS specific calls.

cpu.js

var os = require("os");

//Create function to get CPU information
function cpuAverage() {

  //Initialise sum of idle and time of cores and fetch CPU info
  var totalIdle = 0, totalTick = 0;
  var cpus = os.cpus();

  //Loop through CPU cores
  for(var i = 0, len = cpus.length; i < len; i++) {

    //Select CPU core
    var cpu = cpus[i];

    //Total up the time in the cores tick
    for(type in cpu.times) {
      totalTick += cpu.times[type];
   }     

    //Total up the idle time of the core
    totalIdle += cpu.times.idle;
  }

  //Return the average Idle and Tick times
  return {idle: totalIdle / cpus.length,  total: totalTick / cpus.length};
}

//Grab first CPU Measure
var startMeasure = cpuAverage();

//Set delay for second Measure
setTimeout(function() { 

  //Grab second Measure
  var endMeasure = cpuAverage(); 

  //Calculate the difference in idle and total time between the measures
  var idleDifference = endMeasure.idle - startMeasure.idle;
  var totalDifference = endMeasure.total - startMeasure.total;

  //Calculate the average percentage CPU usage
  var percentageCPU = 100 - ~~(100 * idleDifference / totalDifference);

  //Output result to console
  console.log(percentageCPU + "% CPU Usage.");

}, 100);

I have slightly modified this script to get the average load for a single cpu

cpuIAverage = function(i) {
  var cpu, cpus, idle, len, total, totalIdle, totalTick, type;
  totalIdle = 0;
  totalTick = 0;
  cpus = os.cpus();
  cpu = cpus[i];
  for (type in cpu.times) {
    totalTick += cpu.times[type];
  }
  totalIdle += cpu.times.idle;

  idle = totalIdle / cpus.length;
  total = totalTick / cpus.length;
  return {
    idle: idle,
    total: total
  };
};

and

cpuILoadInit = function() {
  var index=arguments[0];
  return function() {
    var start;
    start = cpuIAverage(index);
    return function() {
      var dif, end;
      end = cpuIAverage(index);
      dif = {};
      dif.cpu=index;
      dif.idle = end.idle - start.idle;
      dif.total = end.total - start.total;
      dif.percent = 1 - dif.idle / dif.total;
      return dif;
    };
  };
};

where

cpuILoad = (function() {
  var info=[],cpus = os.cpus();
  for (i = 0, len = cpus.length; i < len; i++) {
    var a=cpuILoadInit(i)();
    info.push( a );
  }
  return function() {
    var res=[],cpus = os.cpus();
    for (i = 0, len = cpus.length; i < len; i++) {
      res.push( info[i]() );
    }
    return res;
  }

})();

I think it's easier than this, but it should print on a i7:

> cpu.cpuILoad()
[ { cpu: 0,
    idle: 148.75,
    total: 1103.75,
    percent: 0.8652321630804077 },
  { cpu: 1,
    idle: 797.5,
    total: 1103.75,
    percent: 0.2774631936579841 },
  { cpu: 2,
    idle: 142.5,
    total: 1102.5,
    percent: 0.8707482993197279 },
  { cpu: 3,
    idle: 798.75,
    total: 1102.5,
    percent: 0.2755102040816326 },
  { cpu: 4,
    idle: 146.25,
    total: 1102.5,
    percent: 0.8673469387755102 },
  { cpu: 5, idle: 795, total: 1102.5, percent: 0.2789115646258503 },
  { cpu: 6,
    idle: 142.5,
    total: 1102.5,
    percent: 0.8707482993197279 },
  { cpu: 7,
    idle: 796.25,
    total: 1101.25,
    percent: 0.27695800227014755 } ]

So this script seems to loop through all of the cores. Is it possible to shorten it in such a way that it only reads the first core? I'm running on a 1-core VPS, so I don't think there's any advantage for me to run through all cores.

Here is a way that console every single cpu average load:

const os = require('os')

function delta() {
  const cpus = os.cpus()

  return cpus.map(cpu => {
    const times = cpu.times
    return {
      tick: Object.keys(times).filter(time => time !== 'idle').reduce((tick, time) => { tick+=times[time]; return tick }, 0),
      idle: times.idle,
    }
  })
}

let startMeasures = delta()
setInterval(() => {
  const endMeasures = delta()
  const percentageCPU = endMeasures.map((end, i) => {
    return ((end.tick - startMeasures[i].tick) / (end.idle - startMeasures[i].idle) * 100).toFixed(3) + '%'
  })

  console.log(percentageCPU.join(' '), '\n')

  // reset
  startMeasures = delta()
}, 2000)

Hi mate, great gist.
This line of code though seems a bit ambiguous to me:
https://gist.github.com/bag-man/5570809#file-cpu-js-L43
I get that you're trying to get rid of the fractional-part but couldn't you just use Math.floor or Math.rand? I say that only for readability/maintainability reasons.

Noob <, however how can i use this with chart.js?

Here is a way to get the medium load every N milliseconds, checking the load every M milliseconds.
Also, I've used promise style asynch programming.
https://gist.github.com/GaetanoPiazzolla/c40e1ebb9f709d091208e89baf9f4e00

I rewrite it like closure function:

That looks damn beautiful - but I can't get it to work. What's that style called? What kind of processor / compiler do you need for that?

With love, but mine works better: https://gist.github.com/GaetanoPiazzolla/c40e1ebb9f709d091208e89baf9f4e00 <3