Add copySign

snippets/copySign.md

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+---
+title: copySign
+tags: math,beginner
+---
+
+Returns the absolute value of the first number, but the sign of the second.
+
+- Use `Math.sign()` to check if the two numbers have the same sign.
+- Return `x` if they do, `-x` otherwise.
+
+```js
+const copySign = (x, y) => Math.sign(x) === Math.sign(y) ? x : -x;
+```
+
+```js
+copySign(2, 3); // 2
+copySign(2, -3); // -2
+copySign(-2, 3); // 2
+copySign(-2, -3); // -2
+```