Timing a JavaScript function

Question

I'm trying to make a function that will time other JavaScript functions.

Basically, it would do something like this:

var start = new Date().getTime();

// the function

var end = new Date().getTime();
return end - start;

But, I'd like to be able to call it on any function. For example, I could say timeThisFunction(add) or timeThisFunction(subtract) and it would see how long it took the functions add and subtract to run.

Is this possible in JavaScript? I know there's scoping issues that could be involved. What's the best way to implement it?

If you're interested in this for some serious purpose and not just as a learning project, you might want to check out Benchmark.js. — Pointy
– Pointy, Commented Oct 25, 2014 at 16:07
Use performance.now() instead of Date(). It is much more precise. — Dmitry
– Dmitry, Commented Oct 25, 2014 at 16:28

meda · Accepted Answer · 2014-10-25 18:13:46Z

4

If you just want to be able to time a single call to a function, it's pretty easy:

function timer(fn) {
  var start = new Date().getTime();

  fn();

  var end = new Date().getTime();
  return end - start;
}

Passing functions around is no different than passing around any other sort of value.

If you need to time a function that requires parameters, just wrap it in another function:

var millis = timer(function() { theRealFunction(something, 22); });

The extra wrapper function will add a tiny amount of overhead, but it shouldn't matter for most purposes.

Similarly, if you need to time a function that expects this to be bound, you'd bind it before the call:

var millis = timer(someObject.someMethod.bind(someObject));

Now, all that said, if you've got a single JavaScript function call that takes enough time for this mechanism to give meaningful results, then there's probably some serious performance flaw worth investigating.

edited Oct 25, 2014 at 18:13

meda

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answered Oct 25, 2014 at 16:02

Pointy

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3 Comments

Jared Farrish Over a year ago

Hopefully the function doesn't return anything itself... :P

JJJ Over a year ago

Yes, but what about those scoping issues? What if fn uses this?

Pointy Over a year ago

@Juhana then bind it before calling the timer function.

Elena · Accepted Answer · 2014-10-26 16:43:01Z

1

Just as an option, you could make timimng function to perform some work for you (context binding, arguments passing).

As a downside, apply is known to be slower then usual function call, so this will no be 100% accurate, but if what is needed is to compare two different implementations of the same function, then it can be useful.

function timeIt(fn, context) {
        var args = Array.prototype.slice.call(arguments, 2),
            start = new Date().getTime();

        fn.apply(context, args);

        return new Date().getTime() - start;

}

Example usage would be:

    function log(what, smthElse) { //ugly function for demonstration
        console.log(what + ' ' +  smthElse);
    };

    timeIt(log, null, 'hello', 'stranger');

edited Oct 26, 2014 at 16:43

answered Oct 25, 2014 at 16:26

Elena

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