Entry

when passing a variable number of parameters you have explained the use of the code 'arg0 = arg0 == void 0 ? 0 : arg0'. Please explain what it does?

Dec 22nd, 2000 22:31
Juergen Thelen, Peter Parker,

Hi Peter,
To understand the block
  function functionName (arg0, arg1, arg2, arg3) {
    arg0 = arg0 == void 0 ? 0 : arg0;
    ..
  }
you first need to know the functionality of the special operators '?:' 
and 'void'.
The special operator '?:' is just a shortcut form of an if-else-block, 
so instead of writing:
  function functionName (arg0, arg1, arg2, arg3) {
    arg0 = arg0 == void 0 ? 0 : arg0;
    ..
  }
I could also write:
  function functionName (arg0, arg1, arg2, arg3) {
    if (arg0 == void 0)
      arg0 = 0
    else
      arg0 = arg0;
    ..
  }
The void operators task is to evaluate the expression following the 
void keyword. The usage of parantheses surrounding the expression to be 
evaluated is optional, but for now I'll put them back in to emphasize 
the relationship:
  function functionName (arg0, arg1, arg2, arg3) {
    if ( arg0 == void(0) )
      arg0 = 0
    else
      arg0 = arg0;
    ..
  }
The particularity of void is, that it evaluates the given expression, 
but does not return a value as a result. In other words void(0) simply 
stands for 'undefined' here, so I could pseudocode:
  function functionName (arg0, arg1, arg2, arg3) {
    if ( arg0 == 'undefined' )
      arg0 = 0
    else
      arg0 = arg0;
    ..
  }
Now imagine several cases of calling this function:
  Case #1   functionName();
  Case #2   functionName("a", "b");
  Case #3   functionName(null, "b");
Case #1: the function is called without any parameters, hence arg0, 
arg1, arg2 and arg3 will have no values upon function entry. This will 
be detected by the if-block and arg0 will be set to 0 (arg0 = 0).
Case #2: the function is called with two defined parameters, hence upon 
function entry arg0 will hold "a", and arg1 will hold "b". This time 
arg0 is not undefined, so arg0 remains unchanged (arg0 = arg0 = "a").
Case #3: the function is called with the special keyword 'null' and the 
defined parameter "b". Usage of the special keyword 'null' results in 
assigning a null value (meaning no value, not a value of zero!) to the 
first parameter, so arg0 will have no value, and arg1 will be "b" upon 
function entry. The if-block will detect that arg0 has no value, and 
like in Case#1 set arg0 to 0. The null keyword comes handy if some 
argments can or must be skipped, but other arguments have to be set.
Hope this helps.
Juergen