JavaScript Booleans

A JavaScript Boolean represents one of two values: true or false.

Boolean Values

Very often, in programming, you will need a data type that can only have one of two values, like

  • YES / NO
  • ON / OFF
  • TRUE / FALSE

For this, JavaScript has a Boolean data type. It can only take the values true or false.

The Boolean() Function

You can use the Boolean() function to find out if an expression (or a variable) is true:

Example

Boolean(10 > 9)

Or even easier:

Example

(10 > 9)
10 > 9

Comparisons and Conditions

The chapter JS Comparisons gives a full overview of comparison operators.

The chapter JS If Else gives a full overview of conditional statements.

Here are some examples:

Operator Description Example
== equal to if (day == "Monday")
> greater than if (salary > 9000)
< less than if (age < 18)

The Boolean value of an expression is the basis for all JavaScript comparisons and conditions.

Everything With a "Value" is True

Examples

100

3.14

-15

"Hello"

"false"

7 + 1 + 3.14

Everything Without a "Value" is False

The Boolean value of 0 (zero) is false:

let x = 0;
Boolean(x);

The Boolean value of -0 (minus zero) is false:

let x = -0;
Boolean(x);

The Boolean value of "" (empty string) is false:

let x = "";
Boolean(x);

The Boolean value of undefined is false:

let x;
Boolean(x);

The Boolean value of null is false:

let x = null;
Boolean(x);

The Boolean value of false is (you guessed it) false:

let x = false;
Boolean(x);

The Boolean value of NaN is false:

let x = 10 / "Hallo";
Boolean(x);

JavaScript Booleans as Objects

Normally JavaScript booleans are primitive values created from literals:

let x = false;

But booleans can also be defined as objects with the keyword new:

let y = new Boolean(false);

Example

let x = false;
let y = new Boolean(false);

// typeof x returns boolean
// typeof y returns object

Do not create Boolean objects.

The new keyword complicates the code and slows down execution speed.

Boolean objects can produce unexpected results:

When using the == operator, x and y are equal:

let x = false;
let y = new Boolean(false);

When using the === operator, x and y are not equal:

let x = false;
let y = new Boolean(false);

Note the difference between (x==y) and (x===y).

(x == y) true or false?

let x = new Boolean(false);
let y = new Boolean(false);

(x === y) true or false?

let x = new Boolean(false);
let y = new Boolean(false);

Comparing two JavaScript objects always return false.