Q1704 How can I performance test JavaScript?

<script language="JavaScript"><!--
// Define a Test object construtor:

function Test(loop, testFunction, description) {
  this.loop = loop;
  this.invokeFunction = testFunction;
  this.description = description;
  this.execute = runTest;
  this.result = getResult;

  this.elapsedTime = 0;
  this.counter = 0;
}

// Define the Test object execute() method:

function runTest() {
  var start = new Date();
  for (var i=0; i<this.loop; i++) this.invokeFunction();
  var end = new Date();
  this.elapsedTime += (end - start) / this.loop;
  this.counter++;
}

// Define the Test object result() method:

function getResult() {
  return this.description + ' : time taken (ms) : ' + (this.elapsedTime/this.counter);
}

// Define how many times we want to loop around each test:

var loop = 10;

// Create an array of tests:

var tests = new Array(
  new Test(loop, testFunction1, 'multiple document.write'),
  new Test(loop, testFunction2, 'single document.write'),
  new Test(loop, function () { var o = ''; for (var i=0; i<1000; i++) o += i; document.write(o); }, 'anonymous function'),
  new Test(loop, function () {}, 'do nothing')
)

// Comment out any output generated by the running of the tests:

document.write('<!--');

// Run each test:

for (var i=0; i<tests.length; i++) {
  tests[i].execute();
}

document.write('-->');

// Display results of each test:

var output ='';
for (var i=0; i<tests.length; i++) {
  output += tests[i].result() + '\n';
}
alert(output);

//--></script>