Flow

Language name

Flow

URL of example repository

Flow's website code has some example .js files (bad file name extension, I know, but maybe with more support like in this feature request, they might be more inclined to add a unique extension) that use Flow syntax:

https://github.com/facebook/flow/blob/d41d9956a6de8a1ab5fce696d67f23158b1e87c9/website/src/js/parser-playground.js

Most popular extensions

IDE extensions? Here's the extension for VS Code:

https://marketplace.visualstudio.com/items?itemName=flowtype.flow-for-vscode

Detected language

Not sure. Here's sample of code:

class A {
    foo: {...} | null = {any: 'thing'}

    someMethod(options: ?{[string]: mixed}): A {
        return new A
    }
}

URL of syntax highlighting grammar

Here's one:

https://github.com/facebook/flow/blob/main/website/src/try-flow/flow-grammar.json

The Flow playground shows the AST for the previous example.

Here's the same example on AST Explorer, under "JavaScript" which admittedly is a little odd because it isn't JavaScript. I hope they can consider a new default file extension and migrate away from .js because the downsides are notable.