# JavaScript program to find if a number is prime or not

## What is a prime number :

A number is called *prime* if that number is divisible by *1* and the number itself. For example, *2, 3, 5, 7* etc. are prime numbers. In this post, I will show you how to check if a number is *prime* or not in JavaScript with examples.

## Method 1: Using a for loop :

This is the simplest way to check for a prime number. We will write one *loop* that will iterate from *2* to *number/2*. For each iteration, we will check if the current loop iteration number can divide the given number or not. If it can divide, it is not a *prime*. If no number can divide it, it is a *prime* number.

function isPrime(num) { if (num <= 1) return false; if (num == 2) return true; for (let i = 2; i <= num / 2; i++) { if (num % i == 0) { return false; } } return true; } console.log(isPrime(2)); console.log(isPrime(53));

### Explanation :

Here,

– *isPrime* is used to check if a number is prime or not. It takes one number *num* as its parameter and returns one boolean value based on *num* is prime or not.

– If the value of *num* is equal to or less than 1 return *false*. If it is *2*, return *true*.

– The *for* loop checks from *2* to *num/2* and if any number can divide *num*, return *false*.

– Return *true* if the loop ends

### Finding all prime numbers from 1 to 100 :

Let’s use the above method to find out all prime numbers from *1* to *100* :

function isPrime(num) { if (num <= 1) return false; if (num == 2) return true; for (let i = 2; i <= num / 2; i++) { if (num % i == 0) { return false; } } return true; } let result = []; for (let j = 1; j <= 100; j++) { if (isPrime(j)) { result.push(j); } } console.log(result);

Here, we are calling *isPrime* method to check if a number is prime or not. The *for loop* runs from *j = 1* to *j = 100* and adds all prime numbers to the array *result*.

It will print the below output :

[ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97 ]

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