Measuring Function Execution Time in JavaScript

To measure a function’s execution time in JavaScript, you can subtract the timestamp taken before the function runs from the timestamp taken after it finishes. Most people use new Date().getTime() to get a timestamp, but if your function runs in less than 1 millisecond, this won’t work, because the smallest unit of a timestamp is the millisecond. So I recommend the Window.Performance API. It gives you the number of milliseconds since the page started loading, with precision down to a thousandth of a millisecond.

The example code below runs say_hi 100 times and calculates the average execution time (in milliseconds) of those 100 runs.

function timeit(func) {
  var sum = 0;

  for(var i = 0; i < 100; i++) {
    var start = performance.now();
    func()
    var end = performance.now();
    sum += (end - start)/100;
  }

  return sum;
}

function say_hi() {
  console.log("Hi");
}

console.log(timeit(say_hi))