JavaScript判断一个数是否为质数

const readline = require("readline-sync");
console.log("请输入一个数：");
let num = readline.question() - 0;
while (isNaN(num) || num <= 0) {
    console.log("输入有误，请重新输入");
    num = readline.question() - 0;
}
while(num == 1){
    console.log("1既不是质数也不是合数");
    num = readline.question() - 0;    
}

let a = 0;//临时计数用
for (let i = 2; i < num; i++) {
   if (num % i == 0) {
       console.log(num+"不是质数");
       a++;
       break;
   }
}
if (a == 0) {
    console.log(num + "是质数");
}

for (let i = 2; i <= num; i++) {
    if(num % i == 0){
       if (num == i) {
           console.log(num + "是质数");
       }else{
           console.log(num + "不是质数");
           break;
       }
    }
}

let b = 0;
for (let i = 1; i <= num; i++) {
    if (num % i == 0) {
        b++;
    }
}
if(b == 2){
    console.log(num + "是质数");
}else{
    console.log(num + "不是质数");
}