Type-checking annotation in vanilla Javascript

gistfile1.js

// It's possible to add your own quasi language constructs in JS using comments and Function#toString.
// (Doesn't survive minification)
// (You might need to modularize the execution of the annotations)

function checkType(f) {
  return function(a) {
    var type = f.toString().match(/\/\/(.*)\n/)[1].trim();
    if(type !== typeof(a)) throw new Error('Invalid type');
    return f(a);
  }
}

var halve = checkType(function(n) {
  // number
  return n / 2;
});

halve(4); // returns 2
halve('string'); // emits error