JavaScript boolean type
Summary: in this tutorial, you’ll learn about the JavaScript boolean type that has two literal values true
and false
.
Introduction to the JavaScript boolean type
The JavaScript boolean
primitive type has two literal values: true
and false
.
The following example declares two variables and initializes their values to true
and false
:
let completed = true;
let running = false;
Code language: JavaScript (javascript)
The boolean’s literal values are case-sensitive. This means that the True
and False
are valid identifiers but they’re not boolean
values.
JavaScript allows the values of other types to be cast to boolean values. To cast a non-Boolean value to a boolean value, you use the built-in Boolean()
function. For example:
let error = 'An error occurred';
let hasError = Boolean(error);console.log(hasError);
Code language: JavaScript (javascript)
Output:
true
Code language: JavaScript (javascript)
How it works.
- First, declare a variable
error
that holds a literal string'An error occurred'
. - Second, cast the
error
variable to a boolean value using theBoolean()
function. - Third, output the value of the
hasError
variable to the console.
Because the error
variable holds a non-empty string, the Boolean()
function casts its value to true
.
The following table shows how the Boolean()
function casts the values of other types to boolean values:
Data Type | Values converted to true | Value Converted to false |
---|---|---|
string | Any non-empty string | “” (empty string) |
number | Any Non-zero number | 0 , NaN |
object | Any object | null |
undefined | (not relevant) | undefined |
This table is important because some statements automatically cast a non-boolean value to a boolean value using the Boolean()
function.
For example, the if
statement executes a block if a condition is true
. If you use a non-boolean value, it’ll use the Boolean()
function to implicitly cast that value to a boolean value.
Note that you’ll learn about the if
statement in the if
tutorial.
See the following example:
let error = 'An error occurred';if (error) {
console.log(error);
}
Code language: JavaScript (javascript)
Output:
An error occurred
In this example, since the error
variable holds a non-empty string, the if
statement evaluates its value to true
. Therefore, it executes the console.log(error)
to output the error
to the console.
If you change the value of the error
variable to an empty string (""
), you won’t see anything in the output because the if
statement evaluates it as false
:
let error = '';
if (error) {
console.log(error);
}
Code language: JavaScript (javascript)
Summary
- JavaScript
boolean
type has two literal valuestrue
andfalse
. - Use the
Boolean()
function to cast a non-boolean values to a boolean value. - Some statements implicitly cast a non-boolean value into a boolean value.