```
[].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.

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

onlynephew: there are no others to consider.

From the ECMAScript specification of Array.prototype.every (bold emphasis mine):

`every`

callscallbackfnonce for each element present in the array, in ascending order,until it finds one where. If such an element is found, every immediately returnscallbackfnreturns`false`

`false`

. Otherwise, ifcallbackfnreturned`true`

for all elements,`every`

will return`true`

.[...]

`every`

acts like the "for all" quantifier in mathematics.In particular, for an empty array, it returns.`true`

Considering the first bolder phrase above: since `every`

finds no elements for which the callback return `false`

(because the callback never even runs, because there are no elements), it returns `true`

, as confirmed by the second bolded phrase.

It's more mathematically valid to say "every" is - vacuously - true if there are no elements.

You need it so the relationship "for all x, P" is the same as "NOT(there exists x such that not P)".

It's somewhat a matter of convention but it does "make the math work out nicely" quite often.

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.)

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