> For the complete documentation index, see [llms.txt](https://codehs.gitbook.io/introcs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://codehs.gitbook.io/introcs/basic-javascript-and-graphics/booleans.md).

# Booleans

How do we write code that tells us whether a user is logged in to our program? Booleans are the solution to these questions.

## What are booleans?

A boolean refers to a value that is true or false. Those are the only values of a boolean expression, and these are useful if we want to check if something is true or false.

## Meet George Boole

Let's meet the fellow behind the name, "booleans," George Boole. Mr. Boole was an English-born mathematician, philosopher, and logician. He introduced boolean algebra in *The Mathematical Analysis of Logic* (1847) and *Investigation of the Laws of Thought* (1854). Many consider him to be one of the founders of Computer Science.

![George Boole](https://2758402946-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-M4CE_DmHLmpU7V-mTCM%2F-M4CE_o17xJUBglR8D4N%2F-M4CEiSkCwY8M9R1YfFU%2Fjavascript_boole.jpg?generation=1586138909168570\&alt=media)

## Example with booleans

How about an example? Let's create a variable and set it equal to true. Then, we'll print the variable.

We first want to declare the variable, `loggedIn`, and set it to true or false. The box below shows the value of `loggedIn`.

```
var loggedIn = true;
```

![CodeHS](https://2758402946-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-M4CE_DmHLmpU7V-mTCM%2F-M4CE_o17xJUBglR8D4N%2F-M4CEiSmwT17nel8Mmpi%2Fjavascript_boolean_loggedin.png?generation=1586138909354664\&alt=media)

Now, we can go ahead and print it out. We can similarly set the variable to false instead:

```
var loggedIn = false;
```

Notice that we do not need to have quotations around `true` or `false`.

The full example looks like:

```
// In this program we declare a boolean
// value to represent whether the user
// is logged in, and then print it out.
function start(){
    var loggedIn = false;
    println("User logged in?: " + loggedIn);
}
```

Remember, use booleans if you want to set a variable to true or false!