Truthy and Falsey values in JavaScript

Overview

JavaScript has a convention of Truthy and Falsey (sometimes referred to as Falsy).

When JavaScript is dealing with booleans it automatically converts non boolean datatype values to boolean. This is known as type coercion.

Falsey values are datatype values that get converted to false. They are

1. false (boolean)

2. 0 (number)

3. -0 (number)

4. 0n (BigInt)

5. -0n (BigInt)

6. "" (string)

7. null

8. undefined

9. NaN and

10. document.all

    
    

Truthy values are datatype values that get converted to true. They are any value that is not listed above as a falsey value.

There are 8 datatypes in JavaScript, and we will go through truthy and falsey examples of each.

1. Number

2. String

3. Boolean

4. Null

5. Undefined

6. BigInt

7. Symbol

8. Object

Number

Any number is true except 0 and -0.

Truthy and Falsey code output for Number datatypes.

String

All strings are true, except empty strings, even "false" in a string is true.

Truthy and Falsey code output for String datatypes.

Boolean

Simple to remember this one true is true and false is false.

Truthy and Falsey code output for Boolean datatypes.

Nulls

Null is always false.

Truthy and Falsey code output for Null datatypes.

Undefined

Undefined is always false.

Truthy and Falsey code output for Undefined datatypes.

NaN

When passing a string instead of number a NaN datatype is returned. NaN datatypes are always false

Truthy and Falsey code output for NaN datatypes.

BigInt

All BigInt's are true except -0n and 0n.

Symbols

Symbols are always true

Objects

Objects are always true, even empty objects.

Document.All

Documnet.All is always false

Arrays

Arrays are always true.

Comparing Truthy and Falsey values

PLEASE NOTE it is bad practice to not use the strict equality operator, always use ===, the below code is for demonstration purposes only. Stange behavior occurs when using loose equality operator, ==. Many people make the mistake of thinking this is related to truthy and falsey values HOWEVER its due to the complex rules around datatype conversion.

For example

• 1==true will returns true,

• 12==true returns false
  

1==true returns true because the JavaScript conversion rules converts a number of 1 to a boolean of true. It is easy to think this is because 1 is a truthy value.

BUT 12==true returns false because number 12 does not convert to a true boolean value. Therefore, clearly the equality operator does not care about truthy or falsey values.

References

1. Truthy - MDN Web Docs Glossary: Definitions of Web-related terms | MDN

2. Falsy - MDN Web Docs Glossary: Definitions of Web-related terms | MDN

3. Type coercion - MDN Web Docs Glossary: Definitions of Web-related terms | MDN