Understanding undefined in JavaScript

Introduction

In JavaScript, the undefined value is a primitive value that represents the absence of a value. It is one of the six primitive values in JavaScript, along with null, boolean, number, string, and symbol.

When is a variable undefined?

A variable is undefined in the following cases:

• When it has not been declared.
• When it has been declared but not assigned a value.
• When a function is called without arguments and the parameter is not assigned a default value.
• When an object property is accessed and the property does not exist.
• When a class property is accessed and the property does not exist.

Checking for undefined

You can use the typeof operator to check if a variable is undefined. The typeof operator returns a string indicating the type of the variable. For example:


let myVariable;
console.log(typeof myVariable); // undefined


Differences between undefined and null

Both undefined and null represent the absence of a value, but there are some key differences between the two:

• undefined is a primitive value, while null is an object.
• undefined is assigned to variables that have not been initialized, while null is explicitly assigned to variables.
• undefined is a global property, while null is not.

Conclusion

The undefined value is an important part of JavaScript. It is used to represent the absence of a value and can be helpful for debugging and error handling.