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I have seen both spellings of this word, falsy and falsey.

It can mean "something that is equivalent to false" in computer science, such as "The only two falsy values in the Ruby Language are false and nil".

What is the correct usage of this word?

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    In many databases, null is a particular value which is actually nothing at all, not even zero. Similarly false is a Boolean value, which is neither null nor zero. But to work with either null or false they may need to be given a numeric value (generally zero): null, false and zero are "false-like", or falsy. Similarly truthy is true-like: non-null, true or non-zero.
    – St John of the Cross
    Mar 30, 2013 at 23:03
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    But we have heard truthiness recently.
    – GEdgar
    Mar 31, 2013 at 0:10
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    Perhaps a better word is false-like; for example "The only two false-like values in the Ruby Language is false and nil". Certainly I have never heard the term falsy - it just sounds made up to me.
    – Matt
    Mar 31, 2013 at 0:19
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    The only spelling I know for what those two non-words sound like is "falsie": Main Entry: falsie Function: noun Inflected Form:-s Etymology: 1false + -ie : a breast-shaped usually fabric or rubber cup that is used to pad a brassiere — usually used in plural [falsies]" (from M-W 3rd Unabridged Dictionary).
    – user21497
    Mar 31, 2013 at 1:25
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    @Matt the problem with false-like is you've got a really common operator in the middle of the word there. generally you want to keep computer science jargon from breaking the code that it'll be used in.
    – mendota
    Feb 22, 2018 at 22:15

3 Answers 3

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I've always seen falsy and truthy. Falsey is a perfectly acceptable alternative and gives me just as many search results. The word is unfortunately too new to provide good sources. The ECMAScript Language Specification uses “⟦ToBoolean⟧” to refer to the interpretation of of non-Boolean values as Booleans, but makes no use of truthy or falsy.

These terms are widely used in discussions of dynamically typed programming languages such as JavaScript, Ruby, and Python, in which there is a Boolean data type but other values which can also behave like the Booleans true and false. It is a way to distinguish between false the noun and false the adjective:

  • “x is false” (clear)
  • “x is false” (ambiguous: is x false or is it a different false value?)
  • “x is falsy” (clear)

As tchrist points out, Perl is a notable exception because it lacks a Boolean type, so falsy values are just called false, in the adjectival sense.

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    How about truthy vs truethy (or truethey)?
    – ⵗⵆⵗ
    Aug 17, 2015 at 17:13
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    @PeterV well to follow @Webveloper 's reasoning, it should be truθy instead. Er...except that it's not so easy to type :)
    – Andy
    Nov 15, 2016 at 4:33
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    @Webveloper although if compression is used it will probably only add O(1) bytes ;)
    – Andy
    Nov 15, 2016 at 4:34
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    Seems like "falsy" is the preferred way, see the MDN docs: developer.mozilla.org/en-US/docs/Glossary/Falsy
    – dmon
    Jul 12, 2017 at 17:07
  • @Suncat2000 if that's a joke, it's a bad/misleading one. the MDN web docs are a wiki. if falsy is being used on those pages, it's by definition the preferred term of the community.
    – mendota
    Feb 22, 2018 at 22:18
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The only such word currently attested by the OED is the plural noun falsies, whose sense is given as:

A padded brassière; breast-pads.

It has its own Wikipedia entry, one which I must advise you is unlikely to be work-friendly in sensu stricto.

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    For some geeky reason, the computer programming world has long maintained a tradition of using words in new ways, with a studied obliviousness to their prior, rude meanings: for example, 'dump'. 'Falsey' is merely another word in this long, and quite useful, tradition.
    – MarkDBlackwell
    Nov 11, 2014 at 19:11
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    @MarkDBlackwell - Do note that "dump" has a long (and un-treasured) tradition in the computer programming world, likely going back to the mid-60s. And if you think that this use of the word is not "rude" you've never had a 3-foot-high stack of "dump" dropped on your desk Friday afternoon.
    – Hot Licks
    Mar 5, 2015 at 23:16
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    @MarkDBlackwell In addition to Hot Licks' response, I'd say that "dump" in the CS sense, both as noun and verb, is merely another application of its preexisting meanings even without the vulgar one, particularly the ones related to unloading/releasing contents. (For example, "dump truck".)
    – JAB
    Apr 2, 2015 at 14:35
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Given that it is a term with a specific meaning used by programmers, I searched stackoverflow.com to see how that programming community have used it.

  • 9,155 results for falsy (7,674 results for "falsy" in quotes)
  • 3,920 results for falsey (3,302 results for "falsey" in quotes)

This seems to indicate that both versions are in use, and that falsy is preferred.

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  • But I'm sure if you compared the frequency to other similar terms, both spellings would hardly ever be used. I've been in the computer biz for 50 years and I don't recall running across either term.
    – Hot Licks
    Jun 23 at 21:53
  • Some programming languages do not have the concept of "falsy" at all. It is related to the concept of loose vs strict typing. I think that strictly-typed languages (C, Java) might not have a need for the word. I mostly work with loosely-typed languages, such as JavaScript and PHP, and in those communities I have encountered it quite often. People also use alternative phrases such as "evaluates to false".
    – Liam
    Jun 23 at 22:52

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