What does the "percentage sign" mean in the Vue.js source code

Question

I am reading Vue.js's source code(here) on github and stumbled upon something both I don't know and can't find an answer.

A related part from the source:

export function isObject (obj: mixed): boolean %checks {
  return obj !== null && typeof obj === 'object'
}

What is the function or meaning of the "%"?

Answer 1

It's a Flow feature called Predicate Function.

In short, it denotes a function performing a type refinement.

In the specific example, it means that Flow knows that when a value is tested positively using isObject, it will be of type object.

Flow has some baked-in checks to refine a type, including things like obj !== null and typeof obj === 'object', but if you extract it to a generic isObject function returning boolean, Flow won't trust that the function is performing a type refinement, unless you mark it with %checks.

Note that TypeScript has a similar feature called User-Defined Type Guards.