How to Check If Number is Prime in Javascript

In this tutorial, you will learn how to check if number is prime in javascript. The number which is divisible only by itself and 1 is known as a prime number. For example 7, 11, 13, 17, etc.

Detecting whether a number is prime or not is very easy, but it can be tricky if the number is coming from an input field because the value present in the input field is of string type. So first, we have to convert it into a number type using the Number() method and then perform any sort of calculations.

There are numerous ways to find whether a number is prime or not. But for the sake of simplicity, we will create a custom method to verify that. It mainly involves the usage of for loop and modulus (%) operator.

In the following example, we will enter a random number in the input field. Upon click of a button, we will check whether the number is prime or not and display the result on the screen.  Please have a look over the code example and the steps given below.


  • We have 4 elements in the HTML file (div, input, button, and h1). The div element is just a wrapper for the rest of the elements.
  • The innerText for the button element is “Get” and for the h1 element, it is “Result”.
  • We have done some basic styling using CSS and added the link to our style.css stylesheet inside the head element.
  • We have also included our javascript file script.js with a script tag at the bottom.
<!DOCTYPE html>
<html lang="en">

  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <link rel="stylesheet" href="style.css">


  <div class="container">
    <input type="number" placeholder="Enter Number">

  <script src="script.js"></script>

.container {        
    text-align: center;

button {
  margin-top: 10px;
  padding: 10px 20px;

input {
  display: block;  
  margin: 10px auto;
  padding: 10px 20px;

input[type=number]::-webkit-outer-spin-button { 
  -webkit-appearance: none; 
  margin: 0; 


  • We have selected the input element, button element, and h1 element using the document.querySelector() method and stored them in input, btnGet, and output variables respectively.
  • We have attached a click event listener to the button element.
  • In the event handler function, we are getting value from the input element using value property. We are converting it into number type using the Number() method and storing it in the num variable.
  • We have the isPrime() method which will take a number as a parameter. It will verify whether the number is prime or not and will return a Boolean value.
  • We are calling the isPrime() method and passing num as a parameter. Depending upon the returned Boolean value, we will assign “Yes” or “No” to the result variable.
  • We are displaying the result in the h1 element using the innerText property.
let input = document.querySelector("input");
let btnGet = document.querySelector("button");
let output = document.querySelector("h1");

btnGet.addEventListener("click", () => {
  let num = Number(input.value);
  let result = isPrime(num) ? "Yes" : "No";
  output.innerText = result;

function isPrime(num) {
  for (let i = 2; i < num; i++) {
    if (num % i === 0) return false;
  return num > 1;