Currify a function

currify.js

// Transform a function into one which takes partial arguments,
// returning a function which can be invoked with the remaining ones.
function currify(f) {                                                                     
  var len = f.length;                                                                     
                                                                                          
  return function _f() {                                                                  
    var old_args = arguments;                                                             
                                                                                          
    return function() {                                                                   
      var args = [];                                                                      
      for (let i = 0, l = old_args.length;  i < l; i++) args[args.length] = old_args[i];  
      for (let i = 0, l = arguments.length; i < l; i++) args[args.length] = arguments[i]; 
                                                                                          
      return (args.length >= len ? f : _f).apply(this, args);                             
    };                                                                                    
  }();                                                                                    
}                                                                                         

//function add(a, b, c) { return a + b + c; }
//var _add = currify(add);

//console.log(_add(1)(2)(3));
//console.log(_add(1, 2)(3));
//console.log(_add(1)(2, 3));
//console.log(_add(1, 2, 3));

I found that curry can be done with lesser nested functions,
the logic and syntax is simpler and comprehensible (thanks to ES6 :D)
https://gist.github.com/AoiYamada/ae494f6b710f5bd70018a70828c083d8