SOURCE

function sum(...args){
    if([...args].length == 1){
        let sum2 = [...args][0];
        var suum = function (y){
            // console.log(y)
            sum2 += y;
            console.log(`output ${sum2}`)
            return suum;
        }
        // suum.valueOf = function(){
        //     return sum2;
        // }
        return suum;
    } else{
        let sum1 = 0;
        for(var i = 0;i < [...args].length; i++){
            sum1 += [...args][i];
        }
        console.log(`outPut1 ${sum1}`);
        // return sum1
    }
}
sum(1, 2)(3).valueOf()
sum(4)(3, 2, 1)(5, 6).valueOf()