Judgement of Prime Number

Q: User input a number, determine if the number is a prime number

First Try

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function vertify(){
            var num = document.getElementById("num").value;
            console.log(num);
            if(num <= 2){
                alert("Not a Prime Number");
            };
            for(var i = 2 ; i < num; i++){
                if (num % i == 0){
                    alert("Not a Prime Number");
                    break;
                }
                if( i == num - 1){
                    alert("Is a Prime Number")
                }
            }
        }

Feedback

I had trouble of deciding where to put the alert(“Is a Prime Number”), it should be outside the for() loop, but if I put it outside, the statement will still be gone through even when (num % i == 0) and break from that if(). it should not be like this. My solution is to see if i went to the last cycle, aka (i == num - 1), it means the previous outcomes are all negative.
Obviously, it’s not a good solution. We can use ‘flag’ to present the status of num, defalt setting is num is a prime number, if we ever come across any situation deciding it’s not a prime number, we can just change the flag. The final output depends on flag.

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function vertify(){
    var num = document.getElementById("num").value;
    console.log(num);
    var flag = true; 
    
    if (num <= 1){
        flag = false;
    }else{
        for (var i = 2; i < num; i++){
            if(num % i == 0){
                flag = false;
            }

        }
    }

    if (flag){
        alert("Is a Prime Number")
    }else{
        alert("Not a Prime Number")
    }
 }

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