JavaScript Array.prototype vs for loops

aResults.md

[randallkoutnik:~/Dropbox/node/perf]$ node index.js
--- .forEach ---
.forEach took 26.271 ms
for loop took 2.369 ms
--- filter ---
.filter took 6.790 ms
for loop took 4.008 ms

filter.js

'use strict';

var now = require("performance-now");
var _ = require('underscore');


module.exports = function (datums) {
  console.log('--- filter ---');
  var datumsFilter = datums.slice(0);
  var startFilter = now();
  var resultFilter = datumsFilter.filter(function (datum) {
    return datum.num > 0.5;
  });
  var endFilter = now();

  console.log('.filter took', (endFilter - startFilter).toFixed(3), 'ms');

  var datumsLoop = datums.slice(0);
  var startLoop = now();
  var resultLoop = [];
  for (var i = 0; i < datumsLoop.length; i++) {
    if (datumsLoop[i].num > 0.5) {
      resultLoop.push(datumsLoop[i]);
    }
  }
  var endLoop = now();

  console.log('for loop took', (endLoop - startLoop).toFixed(3), 'ms');

  var datumsUnderscore = datums.slice(0);
  var startUnderscore = now();
  var resultUnderscore = _.filter(datumsUnderscore, function (datum) {
    return datum.num > 0.5;
  });
  var endUnderscore = now();

  console.log('underscore took', (endUnderscore - startUnderscore).toFixed(3), 'ms');
}

forEach.js

'use strict';

var now = require("performance-now");
var _ = require('underscore');

module.exports = function (datums) {
  console.log('--- .forEach ---');
  var datumsForEach = datums.slice(0);
  var startForEach = now();
  datumsForEach.forEach(function (datum) {
    datum.firstName = 'Some';
  });
  var endForEach = now();

  console.log('.forEach took', (endForEach - startForEach).toFixed(3), 'ms');

  var datumsLoop = datums.slice(0);
  var startLoop = now();
  var resultLoop = [];
  for (var i = 0; i < datumsLoop.length; i++) {
    datumsLoop[i].firstname = 'Some';
  }
  var endLoop = now();

  console.log('for loop took', (endLoop - startLoop).toFixed(3), 'ms');

  var datumsUnderscore = datums.slice(0);
  var startUnderscore = now();
  var resultUnderscore = _.each(datumsUnderscore, function (datum) {
    return datum.num > 0.5;
  });
  var endUnderscore = now();

  console.log('underscore took', (endUnderscore - startUnderscore).toFixed(3), 'ms');
}

index.js

'use strict';

var forEach = require('./forEach');
var filter = require('./filter');

function genData (numItems) {
  var datums = [];
  for (var i = 0; i < numItems; i++) {
    datums.push({
      id: i,
      num: Math.random(),
      name: 'Kittens'
    });
  }
  return datums;
}

var numToGen = process.env.NUM_ITEMS || 100000;

forEach(genData(numToGen));
filter(genData(numToGen));