재귀

                   -Why use Recursion
                   -The Call Stack 
                   -Helper Method Recursion

재귀를 사용하는 이유

호출 스택

function takeShower(){
    return "Showering!"
}

function eatBreakfast(){
    let meal = cookFood()
    return `Eating ${meal}`
}

function cookFood(){
    let items = ["Oatmeal", "Eggs", "Protein Shake"]
    return items[Math.floor(Math.random()*items.length)];
}
function wakeUp() {
    takeShower()
    eatBreakfast()
    console.log("Ok ready to go to work!")
}

wakeUp()

function countDown(num){
    if(num <= 0) {
        console.log("All done!");
        return;
    }
    console.log(num);
    num--;
    countDown(num);
}
countDown(3)

// Iterative Version
function countDown(num){
    for(var i = num; i > 0; i--){
        console.log(i);
    }
    console.log("All done!")
}

도우미 메서드 재귀

function collectOddValues(arr){

    let result = [];

    function helper(helperInput){
        if(helperInput.length === 0) {
            return;
        }

        if(helperInput[0] % 2 !== 0){
            result.push(helperInput[0])
        }

        helper(helperInput.slice(1))
    }

    helper(arr)

    return result;
}

collectOddValues([1,2,3,4,5,6,7,8,9])