the "is" keyword (type predicates)

To allow for type refinement to be abstracted into functions:

function isNumber(n: unknown): n is number {
  if (typeof n === 'number' && !isNaN(n)) return true
  return false
}

Try

[thecotne]

@JSMonk you should allow developer to specify what he/she wants to refine to so refinement itself can be checked against expectation of a developer

for example

function isNumber<T>(n: T): $Refine<T, number>  {
  return typeof n === 'string'// error
}

function isString<T>(n: T): $Refine<T, string> {
  return typeof n === 'string'// ok
}

declare class Array {
  static isArray<T>(T): $Refine<T, Array>
}

of course inference of $Refine would be super cool but developer needs to be able to specify intent

[trusktr]

That’s a good point. It could be possible to make a mistake in the return statement, and therefore by accident change the type guard that the boolean represents.