Question

What is the purpose of double exclamation in Javascript?

Answer 1

The double exclamation or negation(!!) ensures the resulting type is a boolean. If it was falsey (e.g. 0, null, undefined, etc.), it will be false, otherwise, it will be true. For example, you can test IE version using this expression as below,

let isIE8 = false;
isIE8 = !!navigator.userAgent.match(/MSIE 8.0/);
console.log(isIE8); // returns true or false

If you don't use this expression then it returns the original value.

console.log(navigator.userAgent.match(/MSIE 8.0/)); // returns either an Array or null

Note: The expression !! is not an operator, but it is just twice of ! operator.