Where and how can I use the Boolean function in JavaScript?

JavaScript’s built-in Boolean is one of those things I find myself suggesting in code reviews quite often as of late, so I thought I could share some tips about it with the world.

Using Boolean for truth-checking

The Boolean() function is particularly useful when truth-checking data and probably significantly more readable than the double negation (!!) operation:

let x = 'some-value';

// This doesn't look too nice
if (!!x) {
  // ...
}

// This is a lot more readable
if (Boolean(x)) {
  // ...
}

As you can see in the example above, it serves the exact same purpose and is pretty straightforward to use. Similarly, as Boolean() is itself a function returning a boolean value, you can use it for truth-checking collections, filtering arrays etc.:

const values = [0, 0, 2, 0, 3];
// Use as the callback for Array.prototype.some()
const hasValidValue = values.some(Boolean);
// Use as the callback for Array.prototype.filter()
const nonEmptyValues = values.filter(Boolean);

Handle Boolean objects with care

While the Boolean() function is pretty useful, you might run into some issues with the Boolean object and the new Boolean() constructor. The Boolean object is an object wrapper for a boolean value, but the tricky part is that, as an object, it’s always truthy even if the contained value is false!

let x = new Boolean(false);

if (x) {
  // This code is executed
}

For example, the above code will consider x truthy, even if it clearly contains false as its value. This might some confusing, but you can easily avoid it if you generally avoid using Boolean objects and the new Boolean() constructor, unless you are entirely certain that you need to use it for some reason. I cannot find any scenarios where I would need to use this, to be honest, so it might not be all that common to begin with.