Learning JavaScript Data Structures and Algorithms

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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') : console.log('falsy'); 
} 
 
testTruthy(true); // true 
testTruthy(false); // false 
testTruthy(new Boolean(false)); // true (object is always true) 
 
testTruthy(''); // false 
testTruthy('Packt'); // true 
testTruthy(new String('')); // true (object is always true) 
 
testTruthy(1); // true 
testTruthy(-1); // true 
testTruthy(NaN); // false 
testTruthy(new Number(NaN)); // true (object is always true) 
 
testTruthy({}); // true (object is always true) 
 
var obj = { name: 'John' }; 
testTruthy(obj); // true 
testTruthy(obj.name); // true 
testTruthy(obj.age); // age (property does not exist)