@clutter/wt

What is wt

wt is a JavaScript library to track events in the browser.

It provides a single function wt that can be used as follows:

import wt from '@clutter/wt';
button.addEventListener(
  'click',
  () => wt('event'),
);

When button is clicked, this code will instantiate an <img> tag with the src attribute set to: /track.gif?payload[][kind]=event&payload[][url]=CURRENT_URL&payload[][ts]=1517356691057&agent=Mozilla&rts=1517356691561

On the server, the endpoint track.gif is expected to return a valid image (for instance, a transparent pixel) and to parse all the provided parameters, keeping track of the events sent by wt.

Why use wt

The concept of wt is the same as Google Analytics: to use a transparent pixel to track events in the browser. The main difference is that wt is highly customizable and can send events to your own server, rather than to Google.

How to install and load wt

The easiest way to install wt is through Yarn:

yarn add @clutter/wt

Before calling the wt method, import from the library:

import wt from '@clutter/wt';

How to use wt

Events can be tracked with the function wt(*string* kind, *object* params). For a single event, sent kind to 'event' and pass any parameters you want to track in params. For instance:

wt('event', { action: 'hover' })

will instantiate an <img> tag with the src attribute set to

/track.gif
 ?payload[][kind]=event
 &payload[][url]=CURRENT_URL
 &payload[][action]=hover
 &payload[][ts]=1517356691057
 &agent=Mozilla
 &rts=1517356691561

Breaking the URL down, /track.gif is followed by these query parameters:

• payload[][kind]: the kind of tracking; in this case: 'event'
• payload[][url]: the URL of the page where the event occurred
• payload[][action]: the only param to track in this case: 'hover'
• payload[][ts]: the timestamp when the event occurred (Unix-time milliseconds)
• agent: the User Agent of the request
• rts: the timestamp when the event was sent (that is, when the src attribute of the image was set)

How to track multiple events

Multiple events can be tracked by calling wt several times:

wt('event', { action: 'hover' })
// after 0.2 seconds
wt('event', { action: 'click' })

Under the hood, wt is optimized to deal with multiple events.

Firstly, wt instantiates a new image only the first time wt is invoked. Every other time, wt changes the src attribute without creating a new image.

Secondly, wt bundles multiple consecutive events into one request, if they occur within 1.5 seconds. The goal is to avoid hitting the server too many times. If multiple events are sent in the same request, each request has its own payload. For instance, the URL for the two events above looks like this:

/track.gif
 ?payload[][kind]=event
 &payload[][url]=CURRENT_URL
 &payload[][action]=hover
 &payload[][ts]=1517356691057
 ?payload[][kind]=event
 &payload[][url]=CURRENT_URL
 &payload[][action]=click
 &payload[][ts]=1517356691257
 &agent=Mozilla
 &rts=1517356691561

This format of array in query parameters is particularly optimized for Rails.

How to track a pageview

wt('pageview')

How to change the location of the tracking image

By default, wt expects the image to be located at /track.gif in the same host as the current page. This can be changed by initializing wt with a different trackerUrl before tracking:

wt('initialize', {
  trackerUrl: 'www.my-pixel-endpoint.com/track.gif',
});

How to track more data for each event

Any object can be passed when tracking an event, for instance:

wt('event', {
  action: 'click',
  user: {
    id: 1,
    email: '[e-mail]'
  },
  custom: false,
});

How to submit default data for every event

If part of the payload is the same for every event, it can be set once using set. For instance calling:

wt('set', {
  user: {
   id: 1  
  },
});

ensures that the user ID is sent as part of the payload in any event.

Calling set multiple times adds new values without clearing the old ones. In this sense, set behaves like React.Component().setState(). All the values already set can be cleared with:

wt('clear')

Events

You may want to subscribe to lifecycle events for the tracker.

import wt, { SEND_COMPLETED } from '@clutter/wt'
wt('subscribe', SEND_COMPLETED, () => {
  console.log('Analytics data sent');
});

The lifecycle events are:

• SEND_STARTED: "send:started" - the tracker has started sending data via the tracker pixel
• SEND_COMPLETED: "send:completed" - the tracker has finished sending data via the tracker pixel
• QUEUE_COMPLETED: "queue:completed" - the tracker has finished sending data via the tracker pixel and no additional events are queued
• QUEUE_CONTINUED: "queue:continued" - the tracker has finished sending data via the tracker pixel and some additional events are queued

Calling this method will return an unsubscribe method. You can use it to stop listening to the event.

import wt, { SEND_COMPLETED } from '@clutter/wt'

const unsub = wt('subscribe', SEND_COMPLETED, () => {
  console.log('Analytics data sent - you\'ll only see this once');
  unsub();
});

How to implement the server

In order for wt to fully work, a server must be running that provides a track.gif endpoint. This can be built in Rails, node.js or any other technology. An example is provided inside the repo at examples/index.server.js

Changelog

[v1.0.5] - Thursday, February 22, 2018

• Encoded URL Properly