Add Check If A Number Is Positive Or Negative as a JavaScript til

javascript/check-if-a-number-is-positive-or-negative.md

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+# Check If A Number Is Positive Or Negative
+
+The
+[`Math`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math)
+module has a handy function for checking if a number is _positive_ or
+_negative_. Or _zero_, for that matter.
+
+It is
+[`Math.sign`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/sign).
+
+```javascript
+> Math.sign(5)
+1
+
+> Math.sign(-5)
+-1
+
+> Math.sign(0)
+0
+```
+
+Any positive number will result in `1`. Any negative number will result in
+`-1`. If the number happens to be `0`, then `0` will be returned.
+
+This function goes real well with a switch statement.
+
+Note also that anything that isn't a number will result in `NaN`.
+
+```javascript
+> Math.sign("one")
+NaN
+```