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In JavaScript:
[].every(()=>true) === true
Okay, I've compiled everything into a blog post!
JavaScript WTF: Why does every() return true for an empty array?
https://humanwhocodes.com/blog/2023/09/javascript-wtf-why-does-every-return-true-for-empty-array/
@nzakas there's no check that fails, because the array is empty, so the result is true. The every method checks every array element until one element fails the check, which is the condition for it returning false.
@functionalscript @ziadkh0 @nzakas
Not limited to Javascript; this is true in any language that implements a universal quantification function. (E.g., Iterator::all in Rust.)
@ziadkh0 @nzakas This is consistent with basic first order logic. Let X= Ø (the set of all object is the empty set). Then assume there is a proposition for which it is NOT true that ∀x∈X P(x) (for all x part of X, P(x) is true), that means that ∃x∈X (there exists an x part of X) such that NOT P(x). However, this is a contradiction, since X= Ø (X is the empty set). Hence ∀x∈X P(x), is true for every arbitrary proposition P.
Nice! Array.some works in the opposite manner:
[].every(() => false) === true
[].some(() => true) === false
@nzakas It’s “vacuously true”
Also [].every(x => x === true) === true
The condition doesn’t matter for an empty array!