To tell if a Number in JavaScript is prime, we can use the modulus (remainder) operator **%** along with an **if conditional statement**. We can make a simple function that will let us know if a number is prime or not. First, here is the JavaScript code that determines whether a number is prime or not.

```
for (var i = 2; i < num; i++){
if (num % i == 0){
// Number is NOT prime
}
}
if (num > 1){
// Number IS prime
} else {
// Number is NOT prime
}
```

**num** in the code above would be the number we want to check is prime or not.

We can put this code into a function so that we only have to enter a number into our function to see if it is prime. The function will return **true** if the number **is prime**, and **false** if the number **is not prime**.

```
function isPrime(num) {
for (var i = 2; i < num; i++){
if (num % i == 0){
// Number is NOT prime
return false;
}
}
if (num > 1){
// Number IS prime
return true;
} else {
// Number is NOT prime
return false;
}
}
```

## Tell if a Number in JavaScript is Prime with a Click

Below we will provide code to let the user input a number, and then use the our function above to tell if the number is prime or not. Here is our simple HTML setup:

```
<p>Type a whole number to know if it's prime:</p>
<input id="userVal" type="text" value="">
<div id="click-me" onclick="isPrime()">Get results</div>
<div id="results"></div>
```

First, we will add an **onclick event** to the submit button to run a function we will create.

We will then use the value property along with the **getElementById** method to get the value of the input.

We will then determine if the number is prime using the function we created **isPrime()** above. We will add to our function above so that we can display the results to the user.

Finally, we will display the results using the textContent property.

```
function isPrime(){
//Get the user input
var num = Number(document.getElementById("userVal").value);
//Check if the number is prime and display the results
var result = true;
for (var i = 2; i < num; i++){
if (num % i == 0){
// Number is NOT prime
result = false;
document.getElementById("results").textContent = num + " is NOT Prime.";
}
}
if (num > 1 && result != false){
// Number IS prime
document.getElementById("results").textContent = num + " is Prime.";
} else {
// Number is NOT prime
document.getElementById("results").textContent = num + " is NOT Prime.";
}
}
```

The final code and output for this example are below:

Code Output:

Type a whole number to know if it’s prime:

Full Code:

```
<p>Type a whole number to know if it's prime:</p>
<input id="userVal" type="text" value="">
<div id="click-me" onclick="isPrime()">Get results</div>
<div id="results"></div>
<script>
function isPrime(){
var num = Number(document.getElementById("userVal").value);
var result = true;
for (var i = 2; i < num; i++){
if (num % i == 0){
result = false;
document.getElementById("results").textContent = num + " is NOT Prime.";
}
}
if (num > 1 && result != false){
document.getElementById("results").textContent = num + " is Prime.";
} else {
document.getElementById("results").textContent = num + " is NOT Prime.";
}
}
</script>
```

Hopefully this article has been useful in helping you tell if a number in JavaScript isprime.

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