double exclamation mark js

1document.getElementById("myBtn").addEventListener("dblclick", function() {
2  alert("Hello World!");
3});

1// Converts anything to boolean. 
2
3!!false === false
4!!true === true
5
6!!0 === false
7!!1 === true
8
9!!parseInt("foo") === false // NaN is falsy
10!!-1 === true               // -1 is truthy
11!!(1/0) === true            // Infinity is truthy
12
13!!"" === false              // empty string is falsy
14!!"foo" === true            // non-empty string is truthy
15!!"false" === true          // ...even if it contains a falsy value
16
17!!window.foo === false      // undefined is falsy
18!!null === false            // null is falsy
19
20!!{} === true               // an (empty) object is truthy
21!![] === true               // an (empty) array is truthy; PHP programmers beware!
22

1let a = null;
2const b = a ?? -1;		// Same as b = ( a != null ? a : -1 );
3console.log(b);		  	// output: -1
4//OR IF
5let a = 9;
6const b = a ?? -1;
7console.log(b);  		// output: 9
8
9//PS.,VERY CLOSE TO '||' OPERATION IN FUNCTION, BY NOT THE SAME

1!oObject  // inverted boolean
2!!oObject // non inverted boolean so true boolean representation

1// convert to boolean 
2// The following examples are the only values which result in a false expression
3
4!!0 // false
5!!"" // false
6!!null // false
7!!undefined // false
8!!NaN // false

1const isIE8 = !! navigator.userAgent.match(/MSIE 8.0/);  
2console.log(isIE8); // returns true or false 
3