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javascriptarraysmethods

Why does Array.prototype.every return true on an empty array?

[].every(i => i instanceof Node) // -> true

Why does the every method on arrays in JavaScript return true when the array is empty. I'm trying to do type assertion like so...

const isT = (val, str) => typeof val === str
const nT = (val, str) => !isT(val, str)
const is = {}

is.Undef = (...args) => args.every(o => isT(o, 'undefined'))
is.Def = (...args) => args.every(o => nT(o, 'undefined'))
is.Null = (...args) => args.every(o => o === null)
is.Node = (...args) => args.every(o => o instanceof Node)
is.NodeList = (...args) => args.every(n => n instanceof NodeList)

but these still return true even when no arguments are passed to them.

Solution

  • See the docs

    every acts like the "for all" quantifier in mathematics. In particular, for an empty array, it returns true. (It is vacuously true that all elements of the empty set satisfy any given condition.)

    As an edit, because I looked Vacuous truth up. I understood it from context, but I was interested in the formal definition. This paraphrased quote exemplifies the meaning:

    "You are my favorite nephew" is a vacuous statement if he is the only nephew: there are no others to consider.