Using console.time to profile performance in your node application

June 09, 2016 0 Comments nodejs, Profiling, Javascript
I was writing an express application, when I started to notice one of the routes was taking quite a while to respond to a request. The route was doing some heavy lifting shaping data from an API .

I needed to figure out where the bottleneck was and optimise it.

How do I start to figure out how and where to optimise?

I could run the V8 profiler (, pipe the result into a file, load that into Chrome and look at the metric table and flame graph. But I find this to be too low level when I know roughly the area that the bottleneck is happening in.

Node provides two api's to deal with this. (


Starts a timer that can be used to compute the duration of an operation. Timers are identified by a unique label. Use the same label when you call console.timeEnd() to stop the timer and output the elapsed time in milliseconds to stdout. Timer durations are accurate to the sub-millisecond.


Stops a timer that was previously started by calling console.time() and prints the result to stdout

To use these, put a console.time('label') before the function you want to profile and a console.timeEnd('label') after it and then run your app.

Here's an example of profiling native and Array.prototype.filter against their lodash counterparts on an array of a million values:

const _ = require('lodash');  
const dataset = [];  
for (let i = 0; i < 1000000; i++) {  

const squared = => x * x);  

console.time('lodash map');  
const lodashSquared =, x => x * x);  
console.timeEnd('lodash map');

const filtered = dataset.filter(x => x % 2 === 0);  

console.time('lodash filter');  
const lodashFiltered = _.filter(dataset, x => x % 2 === 0);  
console.timeEnd('lodash filter');

The output to the console looks like this:

$ map: 199.207ms
$ lodash map: 16.889ms
$ filter: 123.355ms
$ lodash filter: 21.634ms

That's a pretty insane time savings on Node v6.1.0. And this could help you squeeze a bit more performance out of your application.

If you're in the browser, some of them have built in similar tools to give you more information. (


Calling this function initiates a JavaScript CPU profile with an optional label. To complete the profile, call console.profileEnd().


Stops the current JavaScript CPU profiling session, if one is in progress, and prints the report to the Profiles panel.

(Photo / CC BY)