Is it okay to use with()?

Question

Once, I saw an example like this:

var a, x, y;
var r = 10;
with (Math) {
  a = PI * r * r;
  x = r * cos(PI);
  y = r * sin(PI / 2);
}

And

Answer 1

It is okay to use any feature of JavaScript, so long as you understand it.

For example, using with you can access existing properties of an object, but you cannot create new ones.

Observe:

var obj = {a:1,b:2};
with(obj) {
    a = 3;
    c = 5;
}
// obj is now {a:3,b:2}, and there is a global variable c with the value 5

It can be useful for shortening code, such as:

with(elem.parentNode.children[elem.parentNode.children.length-3].lastChild.style) {
    backgroundColor = "red";
    color = "white";
    fontWeight = "bold";
}

Because the properties of the style object already exist.

I hope this explanation is clear enough.