In Javascript - value =!!value

This is true in a whole slew of languages. Here's Ruby:

  irb(main):001:0> 3
  => 3
  irb(main):002:0> !3
  => false
  irb(main):003:0> !!3
  => true

And here's C:

  #include<stdio.h>
  main() {
      printf("%d %d %d", 3, !3, !!3);  // 3 0 1
  }

Even the venerable and (relatively) strongly-typed Python falls prey to this:

  >>> 3
  3
  >>> not 3
  False
  >>> not not 3
  True

Basically any language where the ! operator coerces to boolean (or in C's case, the nearest approximation to boolean). Though personally I tend to agree that implicit coercions are undesirable, don't be too quick to cry JAVASCRIPT WUT, as it has quite a lot of precedence. :)

The edited title is very confusing because "- value = !!value" is not correct. Assuming you mean a dash and not a negative, it's still not correct.

I'm sure the mods will change the title to the one from the actual post: "What the Heck is !! in JavaScript?" which is a much better title.

While I get the arguments for not using progressive enhancement in lots of modern use cases, I'm not sure it's wise to require javascript just so the page content can spin into view.

Recently wanted to convert a string of '0' or '1' to a native boolean, shortcut value = !!~~value;

Does JavaScript not have something like "bool(value)"?