Boolean

Properties

Definition

A Boolean is a logical value: true or false. It answers questions like "Is this statement correct?"

> typeof true
"boolean"

> typeof false
"boolean"

Using booleans

Booleans can be created using literals or the Boolean constructor:

// Literal syntax
> true
true

> false
false

// Boolean constructor
> new Boolean(true)
[Boolean: true]

> Boolean(true)
true

Truthy and falsy values

In JavaScript, values are automatically converted to booleans in boolean contexts. Some values are "truthy" (convert to true) and others are "falsy" (convert to false).

Falsy Values

These values evaluate to false:

Value	Type	Notes
false	boolean	The literal false
0	number	Zero
-0	number	Negative zero
0n	bigint	BigInt zero
""	string	Empty string
null	null	Represents "no value"
undefined	undefined	Represents "not defined"
NaN	number	Not-a-Number

Truthy Values

Everything else is truthy, including:

Value	Type	Notes
true	boolean	The literal true
1	number	Any non-zero number
-1	number	Negative numbers
"hello"	string	Any non-empty string
"0"	string	String "0" (string, not number)
"false"	string	String "false" (string, not boolean)
[]	array	Empty array
{}	object	Empty object
function() {}	function	Functions

Operations on booleans

Logical operators: Truth tables

The && operator

Returns the first falsy value, or the last value if all are truthy.

Expression	Result	Explanation
true && true	true	Both values are true
true && false	false	One is false
false && true	false	One is false
false && false	false	Both are false

When used with non-boolean values, && returns the first falsy value or the last truthy value:

Expression	Result	Explanation
"hello" && "world"	"world"	Returns last truthy
"" && "world"	""	Returns first falsy

The || operator

Returns the first truthy value, or the last value if all are falsy.

Expression	Result	Explanation
true || true	true	At least one is true
true || false	true	First value is true
false || true	true	Second value is true
false || false	false	Both are false

When used with non-boolean values, || returns the first truthy value or the last falsy value:

Expression	Result	Explanation
"hello" || "world"	"hello"	Returns first truthy
"" || "world"	"world"	Returns first truthy

The ! operator

Returns the opposite boolean value.

Expression	Result	Explanation
!true	false	Negates the value
!false	true	Negates the value

When used with non-boolean values, ! converts to boolean first, then negates:

Expression	Result	Explanation
!"hello"	false	String is truthy, negated to false
!""	true	Empty string is falsy, negated to true
!0	true	Zero is falsy, negated to true
!1	false	One is truthy, negated to false

Double NOT (!!)

Converts a value to a boolean by negating twice.

Expression	Result	Explanation
!!"hello"	true	Double negation converts to boolean
!!""	false	Double negation converts to boolean
!!0	false	Double negation converts to boolean
!!1	true	Double negation converts to boolean

Comparison operations

Comparison operations return boolean values:

// Equality
> 5 == 8
false

> 5 === 8
false

// Inequality
> 5 != 8
true

> 5 !== 8
true

// Greater than
> 5 > 8
false

// Less than
> 5 < 8
true

// Greater than or equal
> 5 >= 8
false

// Less than or equal
> 5 <= 8
true

Boolean conversion

The Boolean constructor can convert values to booleans:

> Boolean(1)
true

> Boolean(0)
false

> Boolean("hello")
true

> Boolean("")
false

> Boolean(null)
false

> Boolean(undefined)
false

Boolean methods

toString()

Converts a boolean to a string.

> true.toString()
"true"

> false.toString()
"false"

valueOf()

Returns the primitive value of a boolean object.

> const bool = new Boolean(true)
undefined

> bool.valueOf()
true