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snippets/js/s/primes.md

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+---
+title: Primes up to given number
+type: snippet
+language: javascript
+tags: [math,algorithm]
+cover: apples
+dateModified: 2020-12-28T13:49:24+02:00
+---
+
+Generates primes up to a given number, using the Sieve of Eratosthenes.
+
+- Generate an array from `2` to the given number.
+- Use `Array.prototype.filter()` to filter out the values divisible by any number from `2` to the square root of the provided number.
+
+```js
+const primes = num => {
+  let arr = Array.from({ length: num - 1 }).map((x, i) => i + 2),
+    sqroot = Math.floor(Math.sqrt(num)),
+    numsTillSqroot = Array.from({ length: sqroot - 1 }).map((x, i) => i + 2);
+  numsTillSqroot.forEach(x => (arr = arr.filter(y => y % x !== 0 || y === x)));
+  return arr;
+};
+```
+
+```js
+primes(10); // [2, 3, 5, 7]
+```