<div class="hn-table">
<table>
<thead>
<tr>
<td></td><td><strong>Undefined</strong></td><td>Undeclared</td></tr>
</thead>
<tbody>
<tr>
<td>Definition</td><td>A variable that has been declared but has not been assigned a value.</td><td>A variable that has not been declared in any scope using the var, let or const keywords.</td></tr>
<tr>
<td>Behavior</td><td>The variable exists in the current scope and its type is undefined. Accessing it does not cause an error; instead, it returns the special value undefined.</td><td>Attempting to access an undeclared variable will result in a <strong>RefrenceError.</strong></td></tr>
<tr>
<td>scope</td><td>Exists in the scope where it is declared.</td><td>Does not exist in any scope.</td></tr>
</tbody>
</table>
</div><hr />
<p><strong>Undefined:</strong></p>
<pre><code class="lang-javascript"><span class="hljs-keyword">let</span> y;
<span class="hljs-built_in">console</span>.log(y); <span class="hljs-comment">// undefined</span>
</code></pre>
<p><strong>Undeclared:</strong></p>
<pre><code class="lang-javascript"><span class="hljs-built_in">console</span>.log(x); <span class="hljs-comment">// ReferenceError: x is not defined</span>
</code></pre>
<hr />
<details><summary>Why we should know these concepts ?</summary><div data-type="detailsContent">Understanding these concepts helps avoid common pitfalls in JavaScript programming, such as runtime errors and unexpected behaviors.</div></details>

|  | **Undefined** | Undeclared |
| --- | --- | --- |
| Definition | A variable that has been declared but has not been assigned a value. | A variable that has not been declared in any scope using the var, let or const keywords. |
| Behavior | The variable exists in the current scope and its type is undefined. Accessing it does not cause an error; instead, it returns the special value undefined. | Attempting to access an undeclared variable will result in a **RefrenceError.** |
| scope | Exists in the scope where it is declared. | Does not exist in any scope. |

---

**Undefined:**

```javascript
let y;
console.log(y); // undefined
```

**Undeclared:**

```javascript
console.log(x); // ReferenceError: x is not defined
```

---

<details data-node-type="hn-details-summary"><summary>Why we should know these concepts ?</summary><div data-type="detailsContent">Understanding these concepts helps avoid common pitfalls in JavaScript programming, such as runtime errors and unexpected behaviors.</div></details>

In this blog, I will be discussing about the difference between Undefined and Undeclared which new developers get confused between them.

Undefined vs Undeclared [Javascript]

Table of contents

No headings in the article.

	Undefined	Undeclared
Definition	A variable that has been declared but has not been assigned a value.	A variable that has not been declared in any scope using the var, let or const keywords.
Behavior	The variable exists in the current scope and its type is undefined. Accessing it does not cause an error; instead, it returns the special value undefined.	Attempting to access an undeclared variable will result in a RefrenceError.
scope	Exists in the scope where it is declared.	Does not exist in any scope.