Nice. If you wanted to go even more inline:

    Function.prototype.where = function () {
        var fn = this, tests = [].slice.apply(arguments);
        (tests || []).forEach(function(test){
          if (fn.call(this, test[0]) !== test[1]) {
            throw new Error('Failed inline unit test with args: ' + test[0]);
          }
        });
        return fn;
    };
    
      var square = function(n){
        return n * n;
      }.where(
        [[2], 4], // [[arrayOfArguments], expectedResult]
        [[3], 5]  // <-- This test will throw an error
      );