This WhatsApp flaw allows users to track friends, read how

WhatsApp messenger has become a popular way of communicating with friends and family, but did you know that you can also track your friend or even a stranger using the app. According to a blog post by software engineer Robert Heaton, the ‘online status’ feature of the app is a flaw and it can be used to keep a track of the activity timings of the user.
Heaton has also laid the process using which you can track activity of anyone available on WhatsApp. All you need is a laptop, Chrome extension and WhatsApp web. Earlier, Heaton was relying on the last seen option in order to track his friend’s activity, but because of the privacy settings of the app, he was not able to do so.
In the blog post, Heaton has mentioned that he wrote just four lines of Javascript code in order start the tracking the last seen and get the complete pattern of his friend’s activity. He adds, that the company has only given the option to hide the last seen and not the ‘online’ status which enables the user to track anyone.
If you have a number of someone in your mobile and the same number is connected to WhatsApp then you will be able to see if the user is online on the app or not. And it is not necessary for the person who is being monitored to have the contact number of the other person. This means that the even strangers can track your activity using WhatsApp.

Using this method, a user will only be able to track the activity timings of the other user and will not be able to read the WhatsApp messages because of the end-to-end encryption introduced by WhatsApp. Apart from WhatsApp, the Facebook Messenger also has the similar online status issue.