Understanding the Undefined Value in JavaScript

In JavaScript, the undefined value represents the absence of a value. It is a primitive value, meaning that it is not an object and cannot be changed.

When is a Variable Undefined?

A variable is considered undefined in the following situations:

• When it is declared but not assigned a value.
• When a function is called without passing a value for a parameter that is declared as optional.
• When a property is accessed on an object that does not exist.
• When an array element is accessed using an index that is out of bounds.

Checking for Undefined

To check if a variable is undefined, you can use the typeof operator. The typeof operator returns the type of a variable, and for undefined values, it returns the string “undefined”.

const myVariable = undefined;
console.log(typeof myVariable); // Output: "undefined"
  

Comparison with Null

Undefined is often confused with null, but they are two distinct values in JavaScript. Null represents an intentional absence of a value, while undefined represents the lack of a value.

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

Conclusion

Undefined is a fundamental value in JavaScript that represents the absence of a value. It is important to understand when variables are undefined and how to check for them to avoid errors in your code.