for (语句 1; 语句 2; 语句 3) {
    被执行的代码块
}

<button onclick="myFunction()">点击这里</button>
<p id="demo"></p>
<script>
    function myFunction() {
        var x = "";
        for (var i = 0; i < 5; i++) {
            x = x + "The number is " + i + "<br>";
        }
        document.getElementById("demo").innerHTML = x;
    }
</script>

<script>
    cars = ["BMW", "Volvo", "Saab", "Ford"];        //数组
    for (var i = 0, j = cars.length; i < j; i++) {　　//j = 4,如果i<j，name执行for循环，从0开始执行
        document.write(cars[i] + "<br>");
    }
</script>

<script>
    cars = ["BMW", "Volvo", "Saab", "Ford"];
    var i = 2, len = cars.length;
    for (; i < len; i++) {
        document.write(cars[i] + "<br>");
    }
</script>

<script>
    cars = ["BMW", "Volvo", "Saab", "Ford"];
    var i = 0, len = cars.length;
    for (; i < len;) {
        document.write(cars[i] + "<br>");
        i++;    //当我执行第一次循环后i自加1，其实跟放到括号里面差不多
    }
</script>

<button onclick="myFunction()">点击这里</button>
<p id="demo"></p>
<script>
    function myFunction() {
        var x;
        var txt = "";
        var person = {username: "smelond", age: "16", job: "Student"};    //对象
        for (x in person) {    //用in遍历对象的属性
            txt = txt + person[x] + "<br />";
        }
        document.getElementById("demo").innerHTML = txt;
    }
</script>

while (条件){
    需要执行的代码
}

<button onclick="myFunction()">点击这里</button>
<p id="demo"></p>
<script>
    function myFunction() {
        var x = "", i = 0;
        while (i < 5) {
            x = x + "The number is " + i + "<br>";
            i++;
        }
        document.getElementById("demo").innerHTML = x;
    }
</script>

do{
    需要执行的代码
}
while (条件);

<button onclick="myFunction()">点击这里</button>
<p id="demo"></p>
<script>
    function myFunction() {
        var x = "", i = 0;
        do {    //会至少先循环一次
            x = x + "The number is " + i + "<br>";
            i++;    //现在的i=1了
        }
        while (i < 5)    //判断i是否小于5，如果等于5就不执行了，如果while里面写着i<0，当然我们刚刚已经执行了一次了，所以直接停止，并且执行了一次
        document.getElementById("demo").innerHTML = x;
    }
</script>

<script>
    cars = ["BMW", "Volvo", "Saab", "Ford"];
    var i = 0;
    for (; cars[i];) {
        document.write(cars[i] + "<br>");
        i++;
    }
</script>

<script>
    cars = ["BMW", "Volvo", "Saab", "Ford"];
    var i = 0;
    while (cars[i]) {
        document.write(cars[i] + "<br>");
        i++;
    }
</script>

<button onclick="myFunction()">点击这里</button>
<p id="demo"></p>
<script>
    function myFunction() {
        var x = "";
        for (i = 0; i < 10; i++) {
            if (i == 3) {    //小于3都不会执行这个if语句，等于3后执行这个if，会直接跳出for循环
                break;
            }
            x = x + "The number is " + i + "<br>";    //赋值3次
        }
        document.getElementById("demo").innerHTML = x;
    }
</script>

if (i == 3) break;

<button onclick="myFunction()">点击这里</button>
<p id="demo"></p>
<script>
    function myFunction() {
        var x = "", i = 0;
        for (i = 0; i < 10; i++) {
            if (i == 3) continue;    //如果i==3，则跳过，再次执行for循环，从4开始，这句直接省略了大括号{}，因为if语句只有一行代码
            x = x + "The number is " + i + "<br>";
        }
        document.getElementById("demo").innerHTML = x;
    }
</script>

break labelname;

continue labelname;

<script>
    cars = ["BMW", "Volvo", "Saab", "Ford"];    //数组
    list:
    {
        document.write(cars[0] + "<br>");
        document.write(cars[1] + "<br>");
        document.write(cars[2] + "<br>");
        break list;    //直接跳出这个代码块，
        document.write(cars[3] + "<br>");
        document.write(cars[4] + "<br>");    //如果上面没有break list，下面这两句会出现undefined
        document.write(cars[5] + "<br>");
    }
</script>