I was reading react native doc and I came across this line of code
watchID: ?number = null;
.flowconfig file in your project's root directory—that's a dead giveaway your project is using flow.
This specific syntax is saying that if
watchId is passed, it will be a
number type, but the
? indicates that this is also a
maybe type, meaning it may not be passed, in which case the
= null will happen (this is not flow-related), which is how you assign a default value in JS.