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---
title: Extensible prime generator
id: 598ee8b91b410510ae82efef
challengeType: 5
---
## Description
< section id = 'description' >
< p > Write a generator of prime numbers, in order, that will automatically adjust to accommodate the generation of any reasonably high prime.< / p > The generator should be able to : Show the first < b > n< / b > prime numbers.Show the prime numbers in a range.Show the number of primes in a range.Show the < b > n< sup > th< / sup > < / b > prime number.< p > The function should have two parameters. The first will receive < b > n< / b > or the range as an array. The second will receive a boolean, that specifies if the function returns the prime numbers as an array or a single number(the number of primes in the range or the < b > n< sup > th< / sup > < / b > prime). According to the parameters the function should return an array.
< / section >
## Instructions
< section id = 'instructions' >
< / section >
## Tests
< section id = 'tests' >
```yml
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tests:
- text: < code > primeGenerator</ code > is a function.
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testString: assert(typeof primeGenerator === 'function', '< code > primeGenerator< / code > is a function.');
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- text: < code > primeGenerator</ code > is a function.
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testString: assert.deepEqual(primeGenerator(20, true), [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71], '< code > primeGenerator< / code > is a function.');
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- text: < code > primeGenerator</ code > is a function.
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testString: assert.deepEqual(primeGenerator([100, 150], true), [101, 103, 107, 109, 113, 127, 131, 137, 139, 149], '< code > primeGenerator< / code > is a function.');
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- text: < code > primeGenerator</ code > is a function.
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testString: assert.equal(primeGenerator([7700, 8000], false), 30, '< code > primeGenerator< / code > is a function.');
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- text: < code > primeGenerator</ code > is a function.
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testString: assert.equal(primeGenerator(10000, false), 104729, '< code > primeGenerator< / code > is a function.');
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```
< / section >
## Challenge Seed
< section id = 'challengeSeed' >
< div id = 'js-seed' >
```js
function primeGenerator (num, showPrimes) {
// Good luck!
}
```
< / div >
< / section >
## Solution
< section id = 'solution' >
```js
// noprotect
function primeGenerator(num, showPrimes) {
let i,
arr = [];
function isPrime(num) {
// try primes < = 16
if (num < = 16) { return (
num == 2 || num == 3 || num == 5 || num == 7 || num == 11 || num == 13
); }
// cull multiples of 2, 3, 5 or 7
if (num % 2 == 0 || num % 3 == 0 || num % 5 == 0 || num % 7 == 0)
{ return false; }
// cull square numbers ending in 1, 3, 7 or 9
for (let i = 10; i * i < = num; i += 10) {
if (num % (i + 1) == 0) return false;
if (num % (i + 3) == 0) return false;
if (num % (i + 7) == 0) return false;
if (num % (i + 9) == 0) return false;
}
return true;
}
if (typeof num === 'number') {
for (i = 0; arr.length < num ; i + + ) if ( isPrime ( i ) ) arr . push ( i ) ;
// first x primes
if (showPrimes) return arr;
// xth prime
return arr.pop();
}
if (Array.isArray(num)) {
for (i = num[0]; i < = num[1]; i++) if (isPrime(i)) arr.push(i);
// primes between x .. y
if (showPrimes) return arr;
// number of primes between x .. y
return arr.length;
}
}
```
< / section >