Can I set variables to undefined or pass undefined as an argument?

I’m a bit confused about JavaScript’s undefined and null values.

What does if (!testvar) actually do? Does it test for undefined and null or just undefined?

Once a variable is defined can I clear it back to undefined (therefore deleting the variable)?

Can I pass undefined as a parameter? E.g.:

function test(var1, var2, var3) {

}

test("value1", undefined, "value2");