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eg. a sample code from React source code

export function useState<S>(initialState: (() => S) | S) {
  const dispatcher = resolveDispatcher();
  return dispatcher.useState(initialState);
what does <s> here means?
                This is likely not Javascript code, but Typescript code. Is this a .ts file? This sounds like Typescript generics. typescriptlang.org/docs/handbook/generics.html
– Nate
                May 1, 2019 at 1:32
                Given we're talking about the react source, I'd wager it's flow: flow.org/en/docs/types/generics
– HPierce
                May 1, 2019 at 1:41
                If I had any amount of confidence in what that syntax was trying accomplish, I'd write an answer. But unfortunately I'd just have to read the - maybe related - documentation for generics. Either way, I'd bet dollars to donuts that a transpiler (babel) is involved. I'm fairly sure that syntax isn't part of ECMAScript.
– HPierce
                May 1, 2019 at 1:48
It's a generic flow type annotation. It gets added to the code to be able to spot type missmatches using the IDE.  During compile time, these annotations get removed (as they are invalid JS). 
This annotation basically means that you can useState with any type you want, and you could also pass an initializer function that returns a certain type:
  useState("test") // T is string
  useState(1) // T is number
  useState(() => 1) // T is number
        
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