While there is no TI-84 Online Calculator, there are 3 amazing TI-84 Calculator versions that you can immediately use on your computer.
TI-84 Online Calculator Option 1:
TI-84 Plus CE APP for Chrome OS
This is by far the coolest option. Just head over to the Google Webstore at https://chrome.google.com/webstore/detail/ti-84-plus-ce-app-for-chr/compdaiogbfdpildfbleipdcglmmlojo?utm_source=Eloqua and install the ubercool TI 84 PLus CE APP in 1 minutes and you have a fully functional TI 84 running at your finger tips. Once installed you dont have to online in order to use the TI 84 on your computer. Added benefit: You are actually using the latest colorful and most powerful TI-84 on your computer for free. For installation questions and more check out this video:
TI-84 Online Calculator Option 2:
TI-84-Plus-CE SmartView Emulator
Head over to Texas Instruments at https://education.ti.com/en/downloads/trial-software and download the free trial of the TI-84 SmartView Emulator for the TI 84 Plus CE Family. You will love to use the latest TI 84 on your computer. The download and installation of this TI 84 Emulator takes no more than 5 minutes and works like a charm.
TI-84 Online Calculator Option 3:
TI-84 Plus Wabbit Emulator
Our list would not be complete with the popular TI-84 Plus Wabbit Emulator. It has been around for decades, runs on Windows, Linux, Mac OS and Android Devices but not on IOS. Just go to http://wabbitemu.org/ and download the free Wabbit Ti 84 Emulator. It allows you to do everything with your TI-84 on your computer or laptop. It is probably the the most useful App to have when taking math classes in school and college.
In case you need assistance on how to do the installation , click here for a step by step guide : https://www.ti84calcwiz.com/how-to-use-wabbitemu-a-free-ti-84-emulator/