Add js empty states blog

blog_posts/javascript-undeclared-undefined-null.md

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+---
+title: What's the difference between undeclared, undefined and null in JavaScript?
+type: question
+tags: javascript,type
+authors: chalarangelo
+cover: blog_images/river-houses.jpg
+excerpt: JavaScript has three different empty states for variables. Learn their differences and how you can check for each one.
+---
+
+### undeclared
+
+A variable is undeclared if it has not been declared with an appropriate keyword (i.e. `var`, `let` or `const`). Accessing an undeclared variable will throw a `ReferenceError`.
+
+```js
+console.log(x); // ReferenceError: x is not defined
+```
+
+### undefined
+
+A variable is `undefined` if it hasn't been assigned a value. `undefined` is a primitive data type in JavaScript and represents the absence of a value, intentional or otherwise.
+
+```js
+let x;
+console.log(x); // undefined
+```
+
+### null
+
+A variable is assigned a value of `null` like any other value. `null` is also primitive data type in JavaScript and always represents the intentional absence of a value.
+
+```js
+let x = null;
+console.log(x); // null
+```
+
+### Checking for empty states
+
+Undeclared variables will throw an error, which makes them easy to spot and are not very common anyways. `undefined` and `null` can be easily spotted with a conditional as they are both falsy values. Due to that, `null` and `undefined` are loosely equal (`==`), but not strictly equal (`===`).
+
+**Image credit:** [Theodor Lundqvist](https://unsplash.com/@umbriferous?utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText) on [Unsplash](https://unsplash.com?utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText)