The answer
Use null instead of undefined for intentional emptiness. JSON.stringify omits properties with undefined values, but preserves properties with null values as null.
Avoiding undefined Bugs in JavaScript
How do you prevent undefined from breaking JSON payloads in JavaScript?
More FAQs in Avoiding undefined Bugs in JavaScript
Use optional chaining (?.). For example, user?.profile?.name returns undefined if any link in the chain is null or undefined, instead of throwing TypeError.
Because undefined coerces to NaN in numeric contexts. Any arithmetic with NaN produces NaN, which propagates silently. Use default parameters or validate inputs to avoid this.
Use nullish coalescing (??). For example, name = name ?? 'friend' applies the default only when name is null or undefined, not when it is 0 or empty string.
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