Add JS NaN question

blog_posts/javascript-value-not-equal-to-itself.md

@@ -0,0 +1,27 @@
+---
+title: What is the only value not equal to itself in JavaScript?
+type: question
+tags: javascript,type,comparison
+authors: chalarangelo
+cover: blog_images/eagle.jpg
+excerpt: Did you know there's a JavaScript value that's not equal to itself?
+firstSeen: 2021-12-12T05:00:00-04:00
+---
+
+`NaN` (Not-a-Number) is the only JavaScript value not equal to itself when comparing with any of the comparison operators. `NaN` is often the result of meaningless or invalid math computations, so it doesn't make sense for two `NaN` values to be considered equal.
+
+```js
+const x = Math.sqrt(-1); // NaN
+const y = 0 / 0;         // NaN
+
+x === y;                 // false
+x === NaN;               // false
+
+Number.isNaN(x);         // true
+Number.isNaN(y);         // true
+
+isNaN(x);                // true
+isNan('hello');          // true
+```
+
+You can check for `NaN` values using the `Number.isNaN()` function. Note that this is different from the original , global `isNaN()`. Their difference lies in the fact that `isNaN()` forcefully converts its argument to a number, whereas `Number.isNaN()` doesn't. This is why `Number.isNaN()` is considered more robust and preferable in most cases.