The answer
It means a variable exists but has no value. It is the default for declared-but-unassigned variables, missing function parameters, functions with no return, and non-existent object properties.
Why undefined Is a Value in JavaScript
What does undefined mean in JavaScript?
More FAQs in Why undefined Is a Value in JavaScript
Yes. During the memory allocation phase, var variables are allocated memory and set to undefined. It is a real value stored in memory, not a conceptual placeholder.
undefined means the engine has not assigned a value. null means the developer intentionally set the value to empty. typeof undefined is 'undefined'; typeof null is 'object' (a historical bug).
No, it is a property of the global object. In modern JavaScript (ES5+) it is non-writable, so you cannot reassign it. But it is not a reserved keyword like let or const.
Still have questions?
Browse all our FAQs or reach out to our support team
Want to upskill yourself?
Your learning library
Small steps. Big possibilities.
Find your next course. Build something great.