The answer
Check if the parameter === undefined, or use a default parameter. You can also check arguments.length (in regular functions) to see how many arguments were actually passed.
undefined in Function Parameters and Returns in JavaScript
How do you detect a missing argument in a JavaScript function?
More FAQs in undefined in Function Parameters and Returns in JavaScript
The missing parameters are set to undefined. You can handle this with default parameters, which apply only when the argument is undefined or omitted.
No. Defaults apply only when the argument is undefined or omitted. Passing null, 0, or empty string does not trigger the default, because those are real values.
undefined. If a function has no return, or a return with no expression, it returns undefined to the caller.
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