Javascript built-in data structure - Boolean

March 15th, 2020/Share

This tutorial is a part of the Learn everything about Javascript in one course.

Boolean is a data structure to represent logical values. It only has two values: true and false. Boolean is usually an output of logical operator (we will learn more about these operators later) or Boolean() function.

Frequently used Boolean() checks:

Input	Example	Output
Number: none zero `0`	`Boolean(1)`	`true`
Number: zero `0`	`Boolean(0)`	`false`
Number: NaN	`Boolean(NaN)`	`false`
String: none-empty string	`Boolean('a')`	`true`
String: empty string	`Boolean('')`	`false`
`null`	`Boolean(null)`	`false`
`undefined`	`Boolean(undefined)`	`false`
empty array `[]`	`Boolean(![].length)` `Boolean([])` wrong, do not use!	`true`
empty object `{}`	`Boolean(!Object.keys({}).length)` `Boolean({})` wrong, do not use!	`true`

boolean.js

console.log(Boolean(1)) // true
console.log(Boolean(0)) // false
console.log(Boolean(NaN)) // false
console.log(Boolean('a')) // true
console.log(Boolean('')) // false
console.log(Boolean(null)) // false
console.log(Boolean(undefined)) // false

console.log(Boolean(![].length)) // true, empty array
console.log(Boolean([])) // true, empty array, but logic is wrong! DO NOT USE
console.log(Boolean([1,2])) // true, array not empty, DO NOT USE

console.log(Boolean(!Object.keys({}).length)) // true, empty object
console.log(Boolean({})) // true, empty object, but logic is wrong! DO NOT USE
console.log(Boolean({'a': 1})) // true, object not empty, DO NOT USE

// logical operator
console.log(1 > 2) // false
console.log(1 < 2) // true

const myBool1 = true // no "quote" needed
const myBool2 = false // no "quote" needed
console.log(myBool1) // true
console.log(myBool2) // false

Code for this tutorial on github.

Summary

Boolean only has two value true and false.
Special Boolean output table is important to remember.