working-curry.js

let assert = require("assert");

function curryN(N, fn) {
  let self = undefined;
  let collectFn = Object.defineProperties(function (...args) {
    if (this) {
      self = this;
    }
    if (args.length >= N) {
      return fn.apply(self, args);
    }
    else {
      return Object.defineProperties(function (...args2) {
        return collectFn.apply(this, args.concat(args2));
      }, {
        name: {value: fn.name},
        length: {value: N - args.length},
      });
    }
  }, {
    name: {value: fn.name},
    length: {value: N}
  });
  return collectFn;
}

let curry = function (fn) {
  return curryN(fn.length, fn);
};

let sum3 = curry(function mySum(x, y, z) {
  if (this) {
    return [x + y + z, this];
  } else {
    return x + y + z;
  }
});

// TEST LOGIC
assert.equal(typeof sum3(1), "function");
assert.equal(typeof sum3(1, 2), "function");
assert.equal(typeof sum3(1)(2), "function");
assert.equal(sum3(1)(2)(3), 6);

// TEST NAMES
assert.equal(sum3.name, "mySum");       
assert.equal(sum3(1).name, "mySum");   
assert.equal(sum3(1, 2).name, "mySum"); 
assert.equal(sum3(1)(2).name, "mySum"); 

// TEST LENGTHS
assert.equal(sum3.length, 3);
assert.equal(sum3(1).length, 2);
assert.equal(sum3(1, 2).length, 1);
assert.equal(sum3(1)(2).length, 1);

// TEST this BINDING
assert.deepEqual(sum3.bind({foo: "foo"})(1, 2, 3), [6, {foo: "foo"}]);
assert.deepEqual(sum3(1).bind({foo: "foo"})(2, 3), [6, {foo: "foo"}]);
assert.deepEqual(sum3(1, 2).bind({foo: "foo"})(3), [6, {foo: "foo"}]);