What does this expression mean "!!"

Why would somebody use an expression like

if(!!window.JSON) alert('exists');

instead of just

if(window.JSON) alert('exists');

?

Is there another usa开发者_开发技巧ge for the "!!" thing ?

!! will simply cause a boolean conversion, it is not necessary at all to use this construct with the if statement, also in your first snippet, converting an object to boolean makes no sense, it will be always true.

The only values that can coerce to false are null, undefined, NaN, 0, and empty string and of course false anything else will coerce to true.

When it makes sense to use !! is when you want to make sure to get a boolean result, for example when using the Logical && and || operators.

Those operators can return the value of an operand, and not a boolean result, for example:

true && "foo"; // "foo"
"foo" || "bar" ; // "foo"

Imagine you have a function that test two values, and you expect a boolean result:

function isFoo () {
  return 0 && true;
}

isFoo(); // returns 0, here it makes sense to use !!

Edit: Looking at your edit, if (!!window.JSON) is just redundant as I said before, because the if statement by itself will make the boolean conversion internally when the condition expression is evaluated.

They're probably trying to cast the object to a boolean. By applying a boolean operator (!) to the object, it becomes a bool, but they don't actually want the negation, so they apply a second one to cancel the first.

// !! = boolean typecasting in javascript

var one = 1
var two = 2;

if (one==true) app.alert("one==true");
if (!!one==true) app.alert("!!one==true");

if (two==true) app.alert("two==true"); // won't produce any output
if (!!two==true) app.alert("!!two==true");

! is a logical negation, !! is the logical negation applied twice, ie the logical identity

I would assume the code you are reading is just being harmlessly (albeit confusingly) redundant.

Its a boolean conversion, ensuring a conversion to boolean the first time then negating back to the original value.

However it is used incorrectly here and would only really be relevant if you assign it to another variable - for performance reasons

var isJSON = (!!window.JSON);

if (isJSON) doX;
if (isJSON) doY;
if (isJSON) doZ;