What is the purpose of the '!!' operator in JavaScript and what does it do to an expression?

The !! operator in JavaScript is used to convert any value to a boolean type (true or false).

The first ! converts the value to a boolean and inverts it, and the second ! inverts it back, but as a boolean value.

Example:

console.log(!!0);      // false
console.log(!!'text'); // true
console.log(!!null);   // false
console.log(!!{});     // true

This is useful when you need to explicitly obtain a boolean value from any expression.