SOURCE

//1.while循环
var i = 1;
var sum1 = 0;
while(i<=100){
    sum1 = sum1 + i;
    i++;
}
document.write(sum1+"<br/>");

//2.for循环
var i = 1;
var sum2 = 0;
for(;i<=100;i++){
    sum2 = sum2 + i;
}
document.write(sum2+"<br/>");

//3.do-while循环
function sum3(){
    var result = 0;
    var i=0;
    do{
        i++;
        result +=i;
    }
    while(i<=99)
    return result

}
document.write(sum3());