Find All Prime Numbers Between Range in JavaScript | How to Find Prime Numbers in JS

Find all prime numbers between range in javascript. Write a function that finds all prime numbers in a given range using JavaScript. Define two functions, findPrimes and isPrime. findPrimes takes in two arguments, a start and an end number. It will return an array of prime numbers between the start and end. Initialize an empty array called primes. Use a for loop to iterate over the range of numbers from start to end. Lets find all prime numbers between a given range in javascript.

For each number, call the isPrime function, passing in the current loop variable as the argument. If the isPrime function returns true, the current number is a prime number. Add it to the primes array. After the loop, return the primes array.

Now the test of prime number is remaining that we will do in second function. isPrime takes in one argument, a number. It will return a boolean indicating whether or not the number is a prime number.

First check if the number is less than or equal to 1, if so, return false since 1 is not a prime number. Use a for loop to iterate over the range of numbers from 2 to the square root of the input number.

Within the loop, check if the input number is divisible by the current loop variable, if so return false
since a number is not prime if it is divisible by any number other than 1 and itself. If the for loop completes and no divisors were found, return true, indicating that the input number is a prime.

This function will be used by first function. Both are dependent on each other. Call function, with two numbers as arguments for starting and ending point, which will return an array of prime numbers between given range of numbers.

Second method is very special and easy. Define a single function that takes in a single argument 'toNum'. It will return an array of prime numbers up to and including toNum. Use the 'Sieve of Eratosthenes algorithm' to find prime numbers efficiently. This algorithm works by creating an array 'sieve' of Boolean values initialized to false, and then iterating over the range of numbers from 2 to toNum. For each number in the range, if the value of the number in the sieve array is false, it means that the number is prime, add to the primes array. Starting from the number multiplied by 2 and going up to toNum, each multiple of the number is marked as true in the sieve array, which means that these multiples are not prime. After the loop, return the primes array, which contains all prime numbers from 2 to toNum.

Call function with input number, which will return an array of prime numbers to the given limit. So this is how we can find all prime numbers in a given range using JavaScript.

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[dopetag]

I saw your other video called "Check a Number is Prime or Not using Javascript | How to Find Prime Numbers in JS".
In that video, you use different condition for your for loop in isPrime function:
i < num

Could you explain using i <= Math.sqrt(num) in the first approach of this video?

Thank you in advance