## 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 :

Letâ€™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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