What is the difference between undefined and null in JavaScript?
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).
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More FAQs in Why undefined Is a Value in JavaScript
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.
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.
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.
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