The answer
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.
Avoiding undefined Bugs in JavaScript
Why does 5 + undefined give NaN 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.
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.
Because !count catches all falsy values: undefined, null, 0, '', false, and NaN. If 0 is a valid value, use an explicit check like if (count === undefined || count === null) instead.
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