Speaking JavaScript

Chapter 10. Booleans

The primitive boolean type comprises the values true and false:

> typeof false
'boolean'
> typeof true
'boolean'

Converting to Boolean

Values are converted to booleans as follows:

Value	Converted to boolean
undefined	false
null	false
A boolean	Same as input (nothing to convert)
A number	0, NaN → false
other numbers → true
A string	'' → false
other strings → true
An object	true (always!)

Manually Converting to Boolean

There are three ways any value can be converted to a boolean:

Boolean(value)	(Invoked as a function, not as a constructor)
value ? true : false
!!value	A single “not” converts to negated boolean; use twice for the nonnegated conversion.

I prefer Boolean(), because it is more descriptive. Here are some examples:

> Boolean(undefined)
false
> Boolean(null)
false

> Boolean(0)
false
> Boolean(1)
true
> Boolean(2)
true

> Boolean('')
false
> Boolean('abc')
true
> Boolean('false')
true

Truthy and Falsy Values

Wherever JavaScript expects a boolean, you can provide any kind of value and it is automatically converted to boolean. Thus, there are two sets of values in JavaScript: one set is converted to false, while the other set is converted to true. These sets are called falsy values and truthy values. Given the preceding table, the following are all falsy values:

• undefined, null
• Boolean: false
• Number: 0, NaN
• String: ''

All other values—including all objects, even empty objects, empty arrays, and new Boolean(false)—are truthy. Because undefined and null are falsy, you can use the ...