输入一个数计算该数因数的个数：
<input type="text" id="num">
<input type="button" value="因数的个数" id="btn"> -->
<!-- <script>
btn.onclick = function(){
var val = parseInt(num.value);
var i = 1;
var n = 0;
while(i <= val){
if(val % i == 0){
n++;
}
i++;
}
console.log(n);
}
输出100以内素数：
for (var i = 1; i <= 100; i++) {
var n = 0;
for (var j = 1; j < i + 1; j++){
if(i % j == 0){
n++;
}
}
if(n == 2){
console.log(i);
}
}
1-20阶层：
var res = 0;
var sum = 1;
for(var i = 1; i <= 20; i++){
sum *= i;
res += sum;
}
document.write(res + "<br>");
