Learning JavaScript Data Structures and Algorithms - Third Edition

Truthy and falsy

In JavaScript, true and false are a little bit tricky. In most languages, the boolean values true and false represent the true/false results. In JavaScript, a string such as Packt evaluates to true.

The following table can help us better understand how true and false work in JavaScript:

Value Type	Result
undefined	false
null	false
Boolean	true is true and false is false
Number	The result is false for +0, -0, or NaN; otherwise, the result is true
String	The result is false if the string is empty (length is 0); otherwise, the result is true (length >= 1)
Object	true

Let's consider some examples and verify their output:

function testTruthy(val) { return val ? console.log('truthy') ...