4-4while

1、while

 while(表达式){
    代码块
} 
表达式为true, 会一直执行while中的代码块

var a = 0;
while (a <= 3) {
    //只要a<=10为true就一直循环
    alert(a);
    a++;
}

    <script>
        var i=0;
        while(i<=3){
            console.log(i);
            i++;
        }
    </script>