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Question

What are the values in JavaScript that are 'falsey', meaning that they evaluate as false in expressions like if(value), value ? and !value?

There are some discussions of the purpose of falsey values on Stack Overflow already, but no exhaustive complete answer listing what all the falsey values are.

I couldn't find any complete list on MDN JavaScript Reference, and I was surprised to find that the top results when looking for a complete, authoritative list of falsey values in JavaScript were blog articles, some of which had obvious omissions (for example, NaN), and none of which had a format like Stack Overflow's where comments or alternative answers could be added to point out quirks, surprises, omissions, mistakes or caveats. So, it seemed to make sense to make one.

Accepted Answer

Falsey values in JavaScript

false
Zero of Number type: 0 and also -0, 0.0, and hex form 0x0 (thanks RBT)
Zero of BigInt type: 0n and 0x0n (new in 2020, thanks GetMeARemoteJob)
"", '' and `` - strings of length 0
null
undefined
NaN
document.all (in HTML browsers only)
- This is a weird one. document.all is a falsey object, with typeof as undefined. It was a Microsoft-proprietory function in IE before IE11, and was added to the HTML spec as a "willful violation of the JavaScript specification" so that sites written for IE wouldn't break on trying to access, for example, document.all.something; it's falsy because if (document.all) used to be a popular way to detect IE, before conditional comments. See Why is document.all falsy? for details

"Falsey" simply means that JavaScript's internal ToBoolean function returns false. ToBoolean underlies !value, value ? ... : ...; and if (value). Here's its official specification (2020 working draft) (the only changes since the very first ECMAscript specification in 1997 are the addition of ES6's Symbols, which are always truthy, and BigInt, mentioned above:

Argument type Result

Undefined Return false.

Null Return false.

Boolean Return argument.

Number If argument is +0, -0, or NaN, return false; otherwise return true.

String If argument is the empty String (its length is zero), return false; otherwise return true.

BigInt If argument is 0n, return false; otherwise return true.

Symbol Return true.

Object Return true.

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All Falsey Values in Javascript

4 Answers

Falsey values in JavaScript

Sophia Al-Mansoor

Argument type	Result
Undefined	Return `false`.
Null	Return `false`.
Boolean	Return argument.
Number	If argument is `+0`, `-0`, or `NaN`, return `false`; otherwise return `true`.
String	If argument is the empty `String` (its length is zero), return `false`; otherwise return `true`.
BigInt	If argument is `0n`, return `false`; otherwise return `true`.
Symbol	Return `true`.
Object	Return `true`.