curry.js

Function.prototype.curry = function(...args) {
  let fn  = this;
  
  let f = (...restArgs) => {
    if (fn.length < args.length + restArgs.length) {
      args = args.concat(restArgs);
      return fn.apply(this, args.slice(0, fn.length));
    } else {
      args = args.concat(restArgs);
      return f;
    }
  };

  return f;
};

// -----------------------------------
// Example
// -----------------------------------

let foo = function(a, b, c, d, e, f) {
  return Array.prototype.slice.call(arguments).reduce((acc, el) => acc + el,0);
}

let curriedFoo = foo.curry(1, 2, 3, 4, 5, 6);
console.log(curriedFoo());

curriedFoo = foo.curry(1, 2);
console.log(curriedFoo(3, 4)(5, 6));

// ---------------------------------

function curry(fn, ...args) {
  let f = (...restArgs) => {
    if (fn.length < args.length + restArgs.length) {
      args = args.concat(restArgs);
      return fn.apply(undefined, args.slice(0, fn.length));
    } else {
      args = args.concat(restArgs);
      return f;
    }
  };

  return f;
};

// -----------------------------------
// Example
// -----------------------------------

let foo = function(a, b, c, d, e, f) {
  return Array.prototype.slice.call(arguments).reduce((acc, el) => acc + el,0);
}

let curriedFoo = curry(foo, 1, 2, 3, 4, 5, 6);
console.log(curriedFoo());

curriedFoo = curry(foo, 1, 2);
console.log(curriedFoo(3, 4, 5)(6));