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booleans.md

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Booleans

Numbers are a type of data. There are lots of other data types (like undefined). The next data type we'll introduce is called boolean. A boolean has two possible values, true and false.

Try evaluating true in the console. You'll get true. Try assigning the value false to a variable, like let iHatePizza = false. Then evaluate iHatePizza, and you'll see that, that is completely untrue i.e. false.

Conditional expressions

How are booleans useful to us?

Well, there's a type of operator called a ternary, the ternary operator lets you write conditional expressions. Here are a couple examples of ternaries in action:

  • true ? 1 : 0 produces 1
  • false ? 1 : 0 produces 0

The structure of the conditional expression is basically: boolean ? lefthandExpression : righthandExpression. So like true ? 1 : 0. the lefthand expression is evaluated if the condition is true, otherwise the righthand expression is evaluated. If we wrote it in English it would be like: if condition then this expression otherwise this other expression.

We can assign the conditional expression to a variable like any other expression.

// let's organize a party! What food do we need?
let numberOfGuests = 12
let weLovePizza = true
let numberOfPizzas = weLovePizza ? numberOfGuests / 4 : 0
let numberOfSandwiches = numberOfGuests * 2 - numberOfPizzas * 2

If we evaluate the above code, then evaluate numberOfSandwiches we'll get back 12. If we evaluate numberOfPizzas we'll get back 3. So if we have 12 guests, and they like pizza, then we should order 12 sandwiches and 3 pizzas.

What if our guests don't like pizza?

numberOfGuests = 12
weLovePizza = false
numberOfPizzas = weLovePizza ? numberOfGuests / 4 : 0
numberOfSandwiches = numberOfGuests * 2 - numberOfPizzas * 2

Now numberOfPizzas is 0 and numberOfSandwiches is 24.

Notice how we didn't use let in our assignment statements the second time? The let keyword is used to declare a variable. If that variable has already been declared, you can just re-assign the variable to another value. Try declaring a variable twice.

OK, break time.

Comparison operators

A boolean is a data type that has two possible values, true and false. We can use comparison operators to create booleans by comparing other things, like numbers.

operator what it does example
> greater than 2 > 1
< less than 1 < 2
>= greater than or equal to 2 >= 1
<= less than or equal to 1 <= 2
== equal to 2 == 2
!= not equal to 2 != 1
=== strict equal to 2 === 2
!== strict not equal to 2 !== 1

All of the examples in the table above evaluate to true. So 2 > 1 (2 is greater than 1) is true, and 2 < 1 (2 is less than 1) is false.

Logical operators

Booleans can be combined to produce new boolean values.

operator what it does example
&& AND true && true
|| OR true || false
! NOT !false

All of the examples in the table above evaluate to true.

Boolean logic is its own thing, but here's a short intro:

  • AND means if both the left and right sides are true, the expression is true otherwise the expression is false.
  • OR means that if either or both the left and right sides are true the expression is true. If neither is true, the expression is false.
  • NOT negates the value so that true becomes false, and false becomes true.

Of course comparison operators and logical operators are usually combined in an expression to create a boolean value, like this 2 > 1 && -1 < 0 (2 is greater than 1 and -1 is less than 0), which is true.

Exercises

Think through these expressions to decide what value is produced, and then double check in the console:

  • true || false

  • false && false

  • 1 < 2 && 2 > 1

  • 31 < 13 || 1 < 2 && 3 > 1

  • 400 <= 400 && 399 < 400 && (30 > 31 || 400 > 31)

  • true && false && false || false && true

  • true && false || true || false

  • true || false ? true : false

  • true && false && false || false && true ? true && false && false || false && true : 1 < 2 && 2 > 1

Notice the use of parentheses ( ) to group part of the expression?

Examples

// a function for saying whether a number is 0 or not

let isZero = (num) => num === 0

isZero(3) // false
isZero(0) // true
isZero() // false
Note the use of ===. == is used as a comparison operator it coerces the values compared to be comparable. The practical result is that a value like undefined will be coerced to be 0. If you ever forget to give an argument to a function, the parameter will be undefined. so calling isZero() like so, without an argument will result in the comparison undefined === 0, which is false. If we had used ==, the result would have been true. Just something to keep in mind. 
// a function to return the absolute value of a number

let abs = (num) => num < 0 ? num * -1 : num

abs(13) // 13
abs(-13) // 13
// a function that takes a number and enforces a range on it
// returning the limits of the range if the number is outside

let betwixt = (num) => num > 100 ? 100 : num < 0 ? 0 : num

betwixt(56) // 56
betwixt(17) // 17
betwixt(-39) // 0
betwixt(176) // 100

Practice

  • In the callbacks module we modelled finances. Can you add a lottery ticket to finances by using booleans and random numbers?