Question

I read this snippet in the definitive guide:

function not(f)
{
  return function()
  {
    var result=f.apply(this,arguments);
    return !result;
  }
}

What I can’t understand is, since this function f is in the closure, it’s this is already this, why wouldn’t this snippet just directly use var result=f(arguments);?

I even read some calls with undefined/null as the first parameter which I think can completely be replaced with direct call:

...
while(i>len)
{
  if(i in a)
     accumulator=f.call(undefined,accumulator,a[i],i,a);
  i++;
}
...

Why did the author use call() but not direct call? are there any difference between direct function call and call() with undefined as it’s first parameter?

Answer 1

var result=f(arguments);

…Will call f() passing a single argument, the arguments object.

var result=f.apply(this,arguments);

…Will call f() passing the arguments in the arguments object individually (so to speak).

So let’s say f() was defined as:

function f(a,b,c) {
    // do something with a, b, c
    return c;
}

Then given three arguments 1,2,3 the direct call with arguments is like this:

f([1,2,3]);

(Note that arguments is array-like; it isn’t an actual array.)

Whereas the .apply() version is like this:

f(1,2,3);