JavaScript - Is passing undefined as a parameter the same as leaving it out?

Question

Pretty straightforward question. For example, are these equivalent?

window.open('http://someurl.com')

window.open('http://someurl.com', undefined)

Part of me suspects that it might depend on the function itself, but I'm not sure.

Answer 1

In short: In the vast majority of cases, passing undefined is equivalent to leaving the argument out.

However, within a function, you can differentiate between an omitted argument and one that was passed as undefined . While I would never recommend using that distinction to change behavior, you can see the difference between the two if you access the special arguments variable within your function:

 function logNumArgs() { console.log(arguments.length) } logNumArgs() // 0 logNumArgs(undefined, undefined) // 2