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I have seen both versions of the 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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I've never heard the term. We just say false where I come from: "The two false values in Perl are the number 0 and the empty string (that is, the string whose length is 0), as well as anything that evaluates to one of these two, including the undefined value." –  tchrist Mar 30 '13 at 22:37
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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 '13 at 23:03
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But we have heard truthiness recently. –  GEdgar Mar 31 '13 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 '13 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 '13 at 1:25

2 Answers 2

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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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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+1, for "in sensu stricto" :^) –  user19148 Mar 30 '13 at 23:06

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