When should you use null instead of undefined in JavaScript?
Use null when you intentionally want to say 'no value here' (e.g., resetting a variable). Let undefined be the engine's default for things not yet assigned. Use == null to check for both at once.
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More FAQs in undefined vs null in JavaScript
undefined is the engine's default for variables and properties with no value. null is a value the developer assigns to intentionally mean 'no value.' typeof undefined is 'undefined'; typeof null is 'object' (a historical bug).
It is a historical bug from the original implementation. null was represented with a type tag of 0, which the typeof check interpreted as object. It cannot be fixed without breaking existing code, so it remains.
Number(undefined) is NaN, so undefined + 5 is NaN. Number(null) is 0, so null + 5 is 5. This difference is a real source of bugs in arithmetic.
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