if

let score = window.prompt("請輸入分數");

if(score >= 90){
    alert("優秀")
}else if(score >= 70){
    alert("良好")
}else if(score >= 60){
    alert("及格")
}else{
    alert("不及格")
}

switch

    let score = window.prompt("請輸入分數");    
 
    score = Number(score);
    
    switch (true) {
        case score >= 90:
            alert("優秀")
            break;

        case score >= 70:
            alert("良好")
            break;

        case score >= 60:
            alert("及格")
            break;

        default:
            alert("不及格");
    }
 

while

let a = 10 ;
while(a>0){
    console.log(a);
    a--;
}

do while

    let a = 10;
    do {
        console.log(a);
        a--;
    } while (a > 0)

for

    for (i = 0; i <= 20; i++) {
        console.log(i)
    }

for in

一般用來遍歷陣列或物件

    let arr = [3,4,5,6,7,1];

    for(let i=0;i<arr.length;i++){
        console.log(arr[i]);
    }

      <=>

    for(let i in arr){
        console.log(arr[i])
    }

      <=>

    for(let a of arr){
        console.log(a)
    }
 

break

用於強制退出迴圈體

for(var i = 1; i< 10; i++) {
if(i % 4 === 0){
break
}
console.log(i)
}

遇到break，迴圈立即停止，跳出迴圈體

continue

用於強制退出本次迴圈

for(var i = 1; i< 10; i++) {
if(i % 4 === 0){
continue
}
console.log(i)
}   

遇到continue，跳過本次迴圈，繼續下次迴圈