The answer
JSON.stringify omits properties with undefined values, but preserves properties with null values as null. This matters for API payloads where the presence of a null key has meaning.
undefined vs null in JavaScript
How does JSON handle undefined and null differently?
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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