All falsey values in JavaScript

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.

Comparisons with == (loose equality)

It’s worth talking about falsy values’ loose comparisons with ==, which uses ToNumber() and can cause some confusion due to the underlying differences. They effectively form three groups:

• false, 0, -0, "", '' all match each other with ==
  • e.g. false == "", '' == 0 and therefore 4/2 - 2 == 'some string'.slice(11);
• null, undefined match with ==
  • e.g. null == undefined but undefined != false
  • It’s also worth mentioning that while typeof null returns 'object', null is not an object, this is a longstanding bug/quirk that was not fixed in order to maintain compatibility. It’s not a true object, and objects are truthy (except for that “wilful violation” document.all when Javascript is implemented in HTML)
• NaN doesn’t match anything, with == or ===, not even itself
  • e.g. NaN != NaN, NaN !== NaN, NaN != false, NaN != null

With “strict equality” (===), there are no such groupings. Only false === false.

This is one of the reasons why many developers and many style guides (e.g. standardjs) prefer === and almost never use ==.

Truthy values that actually == false

“Truthy” simply means that JavaScript’s internal ToBoolean function returns true. A quirk of Javascript to be aware of (and another good reason to prefer === over ==): it is possible for a value to be truthy (ToBoolean returns true), but also == false.

You might think if (value && value == false) alert('Huh?') is a logical impossibility that couldn’t happen, but it will, for:

• "0" and '0' – they’re non-empty strings, which are truthy, but Javascript’s == matches numbers with equivalent strings (e.g. 42 == "42"). Since 0 == false, if "0" == 0, "0" == false.
• new Number(0) and new Boolean(false) – they’re objects, which are truthy, but == sees their values, which == false.
• 0 .toExponential(); – an object with a numerical value equivalent to 0
• Any similar constructions that give you a false-equaling value wrapped in a type that is truthy
• [], [[]] and [0] (thanks cloudfeet for the JavaScript Equality Table link)

Some more truthy values

These are just a few values that some people might expect to be falsey, but are actually truthy.

• -1 and all non-zero negative numbers

• ' ', " ", "false", 'null'… all non-empty strings, including strings that are just whitespace

• Anything from typeof, which always returns a non-empty string, for example:

  • typeof null (returns a string 'object' due to a longstanding bug/quirk)
  • typeof undefined (returns a string 'undefined')

• Any object (except that “wilful violation” document.all in browsers). Remember that null isn’t really an object, despite typeof suggesting otherwise. Examples:

  • {}
  • []
  • function(){} or () => {} (any function, including empty functions)
  • Error and any instance of Error
  • Any regular expression
  • Anything created with new (including new Number(0) and new Boolean(false))

• Any Symbol

true, 1, "1" and [1] return true when compared to each other with ==.