Those aren't keyword arguments, you are creating variables in the global scope and passing them to the function. a=1 is just saying window.a = 1 and then passing 1 to your function. a is now a variable in global scope.
window.a = 1
console.log(a = 1) // 1
console.log(const a = 2) // error!
a = 1
const a = 2
func(a = 1)
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