JS - The Boolean Constructor - Defining truthy and falsy

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Truthy And Falsy In JavaScript

I am pretty sure that most of you have already heard about the concept of truthy and falsy in JavaScript.

If you haven't, here is a quick dive into the concept:

Every value in JavaScript has an inherent boolean value attached to it. This boolean is used whenever you use the value within a condition, for example, like this:

const value = [];

if (value) { // evaluates to true and the code below runs
  ...
}

Or when double negating a value, which is a shorthand for just calling Boolean(value):

const value = "a string";
const convertedValue = !!value;

or when you use the logical OR to assign a default value:

const value = otherValue || default;

But who or what actually defines which boolean value is attached to which value?

That's the Boolean constructor function!

The Boolean Constructor Function

The Boolean constructor function is a built-in function, which takes an argument and returns a boolean. Luckily for us, we can define a simple input-to-output table and then just have to look up the input we have to find out what result we get.

The Table

Argument TypeValue(s)Result
Undefinedundefinedfalse
Nullnullfalse
Booleantruetrue
falsefalse
Number+0false
-0false
NaNfalse
All other numberstrue
String""false
Any other non-empty Stringtrue
SymbolSymbol("sym"), etc.true
BigInt0nfalse
> 0ntrue
Object[], {}, ...true

How The Runtime Uses The Constructor Function

Whenever you see a statement like the following one:

if (value) {

}

or the negated version:

if (!value) {

}

the runtime actually evaluates them as follows:

if (Boolean(value)) {

}

or

if (!Boolean(value)) {

}

The same applies to the double negation:

const value = !!otherValue;

which is evaluated as

const value = Boolean(value);

Conclusion

That's it. If you ever wonder what the result of a condition that involves non-booleans will be, or what happens if you explicitly convert a value, you can refer to the table above. It lays the foundation for what truthy and falsy in JavaScript is, and is also the foundation for other algorithms JavaScript runtimes use. I am pretty sure that we will discover more of those in the future, together, and we can then refer back to this table, whenever we need it.

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Comments (2)

Ankur Tyagi's photo

Nice work Oliver

Oliver Jumpertz's photo

Hey there, Ankur! Thank you very much! :-)