Update: Whatsapp Web is now officially available for iPhone. The workround provided below for enabling WhatsApp Web on iPhone was more relevant when WhatsApp was not available for iPhone.
Original Article Continues below:
So far there is no official way to use WhatsApp on iPhone. Thankfully, there is a way to enable the WhatsApp Web directly from your iPhone using WhatsApp Web Enabler that works on a jailbroken iPhone.
WhatsApp Web Enabler
Whatsapp Web Enabler has been developed as a workaround by developer iMokhles to enable WhatsApp Web application on iPhone. Officially, there is no way to use WhatsApp Web via an iPhone and there is still a long way before WhatsApp Web becomes available for iPhone.
WhatsApp Web Enabler is available for free from the BigBoss repository in Cydia. The developer has included the ability to disable or enable the tweak from within the Settings app on your iPhone.
How to Use WhatsApp Web with iPhone
1. Download the latest version of WhatsApp for iOS and install it on your iPhone.
2. Download Chrome, Firefox or Opera browser on your computer. Currently WhatsApp Web only works with these browsers and is also supported on Safari (MacOS 10.8+).
3. Install WhatsApp Web Enabler on your jailbroken iPhone – You will need to launch Cydia and look for the WhatsApp Web Enabler tweak and install it on your Phone.
4. Go to Settings app, you should be able to see a ‘WhatsApp Web’ button added there.
5. Find WhatsApp Web Enabler’s preferences, and enable the tweak.
6. Force-close or quit WhatsApp and reopen it again.
7. Tap on Settings tab, select WhatsApp Web and then tap “OK. Got it.”
8. Next from your computer (not your iPhone) go to web.whatsapp.com in your web browser. As mentioned above the browser needs to be Chrome, Firefox or Opera.
9. Once you land at web.whatsapp.com, you will see a QR code. (see below)
10. Scan the QR Code by pointing your iPhone to the computer screen.
11. Once your phone has scanned the code, you will find yourself automatically signed into the WhatsApp Web.
12. Send a text message from your computer to make sure that WhatsApp web is working on your computer.