WDI Fundamentals

WDI Fundamentals Unit 8

Boolean Logic

At this point, we've covered most of what you need to know about basic expressions. However, there are a few quirks and exceptions that we've (until now) glossed over, especially related to Boolean logic. Let's take a closer look at a few:

  • We'll start with undefined and null values. These two values represent a lack of data.
  • Next, we will dive into truthy and falsey — the result of JavaScript's attempt to convert non-Boolean values to Booleans at run time.
  • And, finally: a challenge!


Say you've just opened a JavaScript console. You want to define a new variable but aren't sure what the value is just yet.

var someData;

Without assigning a value to a variable, that variable becomes undefined. We can see this in real time:

// => 'undefined'

Checking to see if a variable is undefined is possible and is an important step, because it allows you to verify that a variable exists before you use it, preventing potential errors. See below:

if (someData === undefined) {
  console.log('No definition found');
  someData = 'We are now assigning a value.';

Another way to check for undefined is to use typeof. This method is possible because undefined is a specific object and its own data type.

// we need a new variable...
var anotherData;
if (typeof(anotherData) === 'undefined') {
  console.log('No definition found');

For extra context, learn more about undefined from Mozilla's MDN entry for "undefined."


null values are values that you decide have no value. Why would I want to do this? Why not use undefined? As it turns out, we can't — variables in JavaScript are only undefined until they've been assigned a value for the first time. null gives us a way to "reset" the value of a variable to "nothing."

Here's an example: Suppose you have an application for keeping track of your possessions. You might have a string called locationOfKeys indicating where you can find your keys. Then, one day, your keys get lost. What's the value of locationOfKeys now? Well, it's "nothing" — they are lost. If null didn't exist, we would have to invent a special string value (perhaps "lost") to signify that the keys are missing. null gives us a standard way of handling that kind of situation in which we can simply say locationOfKeys = null;

That's the purpose of null. It is designed to represent the lack of a value. Whenever variables are defined without any value, they are undefined. This can become tricky to troubleshoot over time, as it acts as a catchall for everything without a value. We can specify our variables as null to represent that there is no data.

// we will define a variable with no value, or null.
var playerScore = null;

We can then evaluate if our value is null:

playerScore === null // 'The player has not scored anything.'

What if we are provided with an input representing a player's actions in a timed game? In Dance, Dance, Revolution, a player must perform an action at a precise interval. It streams a set of commands a player must dance to. These are timed to music. The actions stream across the screen, and, as they pass by the middle of the screen, the player must perform an action in order to score a point. The game can evaluate the player's score in real time!

var userInput = null;
userInput === null // no points... this time

"Falsey" / "Truthy"

We've seen in our first few lessons that some operators can behave differently depending on the kind of values on which they are operating. For example, in the expression 1 + 2 + 3 + 4, the + operator is performing addition; however, in the expression 'Happy birthday, ' + 'Tom', the + operator is working with strings, so it performs a concatenation instead.

Another example of operators with value-dependent behavior are the logical operators:

  • NOT(!)
  • OR (||)
  • AND (&&)

Although they're primarily used with Boolean values, logical operators can also accept inputs that are strings and numbers, among others. When this happens, the logical operators categorize their inputs as being either "falsey" or "truthy." But what do these words mean?

  • To be falsey means that Boolean operators act in mostly the same way that they would if the value was false. For instance, using ! (or, NOT) on a "falsey" value will evaluate to true, just as if you'd written !false. The "falsey" category of values includes the number 0, empty strings (''), null, and undefined. false is, of course, also "falsey."

  • Being truthy is exactly the opposite of being "falsey;" using ! (or, NOT) on a "truthy" value would evaluate to false. Anything that isn't "falsey" is "truthy," including all strings, all non-zero numbers, and (of course) true.

As they say, "To a hammer (read: Boolean operator), everything looks like a nail (read: Boolean value)."

Below are the exact rules that the Boolean operators follow when dealing with non-Boolean input values:

"Falsey" "Truthy"
false true
0 All numbers except 0, including infinity (what you get after dividing by 0)
Empty strings ('') All non-empty strings
Undefined, Null, and NaN (Not a Number, a special type of numeric value) Pretty much everything else

In JavaScript, the Boolean operators !, ||, and && follow the rules below to determine how to behave:

  • NOT: If the input is "truthy," return false; if the input is "falsey," return true.
  • OR: Return the first "truthy" value; if both values are "falsey," return the last "falsey" value.
  • AND: Return the first "falsey" value; if both values are "truthy," return the last "truthy" value.

Take a look at the following table, and see if you can predict the results in the last three columns given the values for A and B in each row.

False False False False True
False True False True True
True False False True False
True True True True False

Test Yourself

Can you predict how the following expressions will be evaluated? Check your answers in JS Bin.

  • 1 || true
  • 3 || null
  • !('')
  • false && undefined
  • true && !0
  • null || 3

Think about your memory game for a moment. How could you use this knowledge to compare cards?

Here's another exercise for you — give it a shot.