Contractual programming in js concept

contract.js

const DEV = true;

function contract(conditions) {
  if (DEV) {
    return function(fn) {
      return function() {
        if (!conditions.pre.apply(null, arguments)) {
          throw 'precondition failed on ' + fn.name
        }

        var result = fn.apply(null, arguments);

        if (!conditions.post(result)) {
          throw 'postcondition failed'
        }

        return result
      }
    }
  }
  return function(fn) {
    return fn;
  };
}

(function test() {
  function testedFunction(param1, param2) {
    return param1 + param2;
  }

  var testedFunction = contract({
    pre: function(param1, param2) {
        return (typeof param1 === 'number'
          && typeof param2 === 'number')
    },
    post: function(result) {
      return result < 12;
    }
  })(testedFunction);

  var result = testedFunction(1, 2);

  console.log(result);
})();