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

In his excellent book "Javascript: The Good Parts", Douglas Crockford lists the "with Statement" in Appendix B: The Bad Parts.

He says "Unfortunately its results can sometimes be unpredictable, so it should be avoided".

He goes on to give an example, where an assignment inside the with will operate on different variables depending on whether the object is defined or not.

See With statement considered harmful (but less detailed than the explanation in the book).