JS Meatballs Pt. 6: Truthy, Falsey and Type Coercion 😇

Truthy and Falsey

I think this point is a good time to step back from the syntax of JavaScript, and take a look into one of its quirks. As well as a type (string, number etc.), every value in JavaScript also has an inherent boolean value (true or false). We can see this by running the following:

if ('hey') {
	return 'Party on!';
}
> Party on! 

In the example above, ‘hey’ evaluates to true. We say it is truthy.

The following values in JavaScript have a false boolean value or are falsey:

• false
• 0 (zero)
• '' or "" (empty string)
• null
• undefined
• NaN

Everything else is truthy!

So why is this even a thing? Well, it allows us to write terse JavaScript. Take this example for our online store.

let itemsInStock = 0

if(itemsInStock) {
	return 'Item is available';
} else {
	return 'Item is not available';
}
> Item is not available'

The if statement takes the itemsInStock variable which has a value of 0. The conditional will then look like this if(0). As we know, 0 is falsey, so the first condition evaluates to false and the else condition is run.

Isn’t that nice? In this example it saves us from writing out if(itemsInStock === 0).

Terse AF.

Type coercion

Sometimes in JavaScript we need to convert a value of one data type to another data type. For example, just say we get an input from an HTML form that contains a number as a string. We can use the built-in Number() method to explicitly convert the string into a number. We can see it convert the age variable below by outputting the type of the variable with the typeof keyword:

let age = "23";
console.log(age, typeof age);
> 23 string

let ageAsNumber = Number(age);
console.log(ageAsNumber, typeof ageAsNumber);
> 23 number

There are three kinds of type coercion. These are:

• Number
• String
• Boolean

Let’s take a look at this example:

console.log("0" + 18);
> 018

You can probably guess what is going on here. Because the first value in the equation is a string and the second is a number, JavaScript is trying to make life easier for us by coercing the number 18 into a string. Any data type (number, string, boolean, null, undefined or symbol) is a valid subject for type coercion.

We can coerce a type into a boolean value using the not operator (!):

let food = 'apple';
!food;
> false

We can coerce it directly into its inherent boolean value by using the not operator twice:

let animal = 'koala';
!!animal;
> true