Local variable vs parameter

Question

function doIt(param) {
   var localVar = param;
   //do lots of stuff with localVar
}

function doIt(param) {
   //do lots of stuff with param
}

Is there any difference in terms of efficiency between the code above?

Answer 1

param variable is already a local variable so the only difference between those two code snippets is that the first one creates useless copy of param variable.

Answer 2

There is no difference. A parameter is just a local variable which is initialized with the passed argument at invokation time.

However, if you are going to change the value of your variable, it is often considered a good practice to leave parameter variables unaltered, simply for readability and maintainability reasons.