The answer
Use strict equality: x === undefined. Avoid if (!x) because it also catches 0, '', null, false, and NaN. Use x == null if you want to check for both null and undefined.
Why undefined Is a Value in JavaScript
How do you check if a variable is undefined in JavaScript?
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.
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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