You may have seen code that looks something like this:

completed = !completed;

No, the developer wasn’t just really excited to be writing JavaScript, though I wouldn’t blame them. That exclamation mark is called the “logical not operator” in JavaScript.

There are a couple of different uses for it. We can see in the above example that the developer wanted to simply toggle the value of completed. The expression above would have been evaluated, completed would have been read as a “truthy” or a “falsy”, then the ! would have inversed the Boolean value of completed. So if it started off true, it would have been flipped to false, and vice-versa. The logical not operator forces the variable it’s in front of to be read as a boolean. An excellent read on “truthy” and “falsy” in JavaScript.

Let’s look at a different example. Say we want to read the value of an input when the user submits a form, and display an alert if they didn’t fill it in:

document.querySelector('#myForm').addEventListener('click', function(){
	var name = document.querySelector('#name').value;
	if(!name){
		return alert('Whoa there.');
	}
	// do something delightful with this user's name
});

If name holds a value, such as the string “Jack Bauer”, then the if check would not pass and we can carry on with the rest of our code. However, if the name was evaluated to be a “falsy”, then we stop execution with our return statement and alert to the user that there was a problem. So, the flow goes: read value of name, it’s an empty string, which means it’s a “falsy”, the presence of the ! means that we inverse the false to a true, and our code inside the if gets executed.

Sometimes, you may across someone really excited, and they’ve used two logical not operators. Something like:

var foo = function(){
	return "bar";
}
if(!!foo){
	console.log("Houston, we have a foo");
}

This is nothing too fancy, the presence of the extra ! simply means that it’s another logical not operator and it flips the inversed boolean back to it’s first value. Without the double logical not operator, the if would have still evaluated the expression to be true, because foo is a value, but it wouldn’t be a real boolean.

Simply put, it’s a shorthand way of casting a value to a Boolean. Shorter than writing:

if(Boolean(foo)){
	console.log("Houston, we have a foo");
}

A simple way to remember it is, “bang bang, you’re boolean”.