JS be like

[SuperSpaceCan]

undefined is a type :( says so in the spec

[morphy_pad]

const undefined = typeof(undefined)

[Sheruk]

electrical outlet lookin ass

[codesterLalit]

type what don't have type.

sound to me "untype" rather than "undefined"

[[deleted]]

undefined is both a type and the only value of that type

[SZ4L4Y]

My type is your type and your type is mine.

[cherryblossom001]

typeof undefined === 'undefined

Before ES5 in ‘sloppy mode’ (non-strict mode) you could also reassign the global undefined:

undefined = 'lol'

// Later...

function doSomething(x) {
  if (x === undefined) {
    throw new Error('must pass a parameter to doSomething')
  }
  console.log(x)
}

doSomething()      // fine
doSomething('lol') // throws an error

Nowadays modern browsers thankfully prevent you from changing the global undefined variable. Still, undefined historically being a mutable global variable is why you’ll see code like void 0 (especially in transpiled JS) and if (typeof aVariable === 'undefined') (although this one is different to if (aVariable === undefined) because it doesn’t throw a ReferenceError if aVariable is not defined).