What does the double exclamation marks (!!) means in Javascript?

Javascript

As we all know, JavaScript is not a static language, but a dynamic language. That means a variable can reference or hold a value of any type and the type can be changed at any point. To better understand the double exclamation marks purpose let's consider you want to convert to a boolean the presence of a value:

const user = 'Doru';
console.log(typeof user); // => string

But we want a boolean, therefore a true/false only. Not truthy, but true. Not falsey, but false! We can sort that with a function:

function isUserPresent(user) {
  if(user) {
    return true;
  }
  return false;
}

isUserPresent('Doru'); // => true
isUserPresent(''); // => false
isUserPresent(0); // => false

Neat! But that is way too bulky. Let's use the !! magic:

const user = 'Doru';
console.log(!!user); // => true

const anotherUser = null;
console.log(!!anotherUser); // => false

Simple, concise, and effective.

Edit