customBind

Function.prototype.bindCustom = function(thisObj, ...args) {
  const boundFunc = (...argsInner) => this.call(thisObj, ...args, ...argsInner);
  return boundFunc;
}

function add(a, b) {
  return this.x + a + b;
}

console.log(add(1, 2)); // NaN

const boundAdd = add.bind({ x: 1 })
const customBoundAdd = add.bindCustom({ x: 1 })
console.log(boundAdd(1, 2)); // 4
console.log(customBoundAdd(1, 2)); // 4

console.log(boundAdd.call({ x: 2 }, 1, 2)); // still 4
console.log(customBoundAdd.call({ x: 2 }, 1, 2)); // still 4

console.log(boundAdd.bind({ x: 2 })(1, 2)); // still 4
console.log(customBoundAdd.bindCustom({ x: 2 })(1, 2)); // still 4

const boundAddWithArg = add.bind({ x: 1 }, 10)
const customBoundAddWithArg = add.bindCustom({ x: 1 }, 10)
console.log(boundAddWithArg(2)); // 13
console.log(customBoundAddWithArg(2)); // 13