Reuters
Facebook CEO Mark Zuckerberg makes his keynote speech during Facebook Inc's annual F8 developers conference in San Jose
If you've ever liked a status on Facebook, tweeted from your phone, checked off an item on your to-do list on Asana, or pinned an image on Pinterest, then you've crossed paths with React.
Today, many core features of some of the most popular apps today are quietly underpinned by React, a framework that was originally built by Facebook engineers and released as open source in 2013.
React is a tool for building user interfaces (UI), which is to say, what an application looks like and how people interact with it.
It's built on JavaScript, the most popular programming language according to the mega-popular code hosting site GitHub, which keeps tabs on such things. Users can reuse components that have already been built, such as buttons.
React started within Facebook, and the team used it for two years before releasing it as open source. That move opened up the doors to allowing anyone to use, modify, or download it for free. In the years since, React has become one of the most popular tools on the web, and is used by companies like Airbnb, Twitter, Uber, Asana, and Pinterest.