SOURCE

function add() {
    let args = [].slice.call(arguments)
    return function newAdd() {
        let sum = 0
        let newArgs = [].slice.call(arguments)
        if (newArgs.length === 0) {
            args.forEach((val) => {
                sum += val
            })
            return sum
        }
        else {
            args = args.concat(...newArgs)
            return newAdd
        }
    }
}
let a = add(1, 2)(3, 4)(5)
console.log(a())