Understanding the Undefined Value in JavaScript

In JavaScript, the undefined value represents the absence of a value. It is a primitive value, and it is distinct from the null value, which represents a value that was explicitly set to null.

When is a value undefined?

A value is undefined in the following cases:

• When a variable is declared but not assigned a value.
• When a function is called without any arguments.
• When an object property is accessed and the property does not exist.
• When an array element is accessed and the index is out of bounds.

How to check for undefined

You can use the === operator to check if a value is undefined:

“`javascript
const myVariable = undefined;

if (myVariable === undefined) {
// The variable is undefined
}
“`

The difference between undefined and null

The undefined value is different from the null value. Null is a value that was explicitly set to null, while undefined is a value that was never assigned a value.

The following table summarizes the key differences between undefined and null:

| Feature | Undefined | Null |
|—|—|—|
| Type | Primitive value | Object |
| Default value | Not assigned | Explicitly set to `null` |
| Equality | `undefined === undefined` is `true` | `null === null` is `true` |
| Strict equality | `undefined === null` is `false` | `null === undefined` is `false` |

Conclusion

The undefined value is a fundamental part of JavaScript. It is important to understand how undefined works and how to check for it in your code.