Proposal: Run-time Type Checks

[srijs]

Hi there!

I am writing a lot of code in an environment where JavaScript and TypeScript freely interact with each other. One thing that we continue to run into is where we would write a function in TypeScript, say:

export function add(x: number, y: number): number {
  return x + y;
}

This function is callable from JavaScript, where we're allowed to do things like:

add('abc', undefined)

This has lead us to introduce type checks for all exported functions in TypeScript, although they really feel verbose in a typed language.

export function add(x: number, y: number): number {
  if (typeof x !== 'number' || typeof y !== 'number') {
    throw new TypeError('x and y need to be numbers');
  }
  return x + y;
}

My idea is to provide an optional flag, say --insertRuntimeTypeChecks, that makes the compiler insert these kinds of checks for function arguments.

The feature will probably be a bit more complicated when thinking about structural types, etc., especially in interplay with classes (it is my understanding that classes are type-checked structurally, not nominally, is that correct?).

But even having it only for primitive types (checking for object, string, number, etc.) could provide a lot of benefits for those of us who write code that interacts with JavaScript, especially should #7140 land, to spare us from manually writing if (x === null) ....

I'd be interested in giving implementing this a shot, if there's interest from the maintainers.

[wizzard0]

I would say a compiler flag is too broad, and a decorator like @runtimeChecked function() {...} would be much less intrusive - also, in this way we also get a place to specify the error message, and whether we allow to remove the typecheck if the compiler decides it is statically not needed.

[wizzard0]

...also, I think you could write said decorator yourself, with type guard functions, except that you will likwly have to specify types explicitly, e.g @runtimeChecked<number,number,number> function(){ ...}

[dsilva]

Babel provides something along the lines of this request: https://github.com/codemix/babel-plugin-typecheck (linked from the same request for Flow: facebook/flow#897 )

[RyanCavanaugh]

Duplicate of #1573