urtype() is a one-line wrapper for Object.prototype.toString.call() that extracts the second word as the "urtype" (Anglophone misappropriation of Germanic prefix -- guilty as charged)

urtype.js

const urtype = (x) => Object.prototype.toString.call(x).split(' ')[1].slice(0, -1)

// examples
[
  ['undefined primitive', undefined],
  ['undefined via void operator', void ''],
  ['null', null],
  ['boolean primitive', true],
  ['number primitive', 123],
  ['string primitive', 'I feel pretty'],
  ['symbol primitive', Symbol('Oh so pretty')],
  ['modest BigInt', 1n],
  ['cheeky BigInt', BigInt(Number.MAX_SAFE_INTEGER) + 1n],
  ['object', {foo: 'bar'}],
  ['array', ['I', 'feel', 'pretty', 'and', 'witty', 'and', 'gay']],
  ['Map', new Map()],
  ['Int8Array', new Int8Array()],
].forEach(([label, testInput]) => console.log(`${label} : ${urtype(testInput)}`))

// undefined primitive : Undefined
// undefined via void operator : Undefined
// null : Null
// boolean primitive : Boolean
// number primitive : Number
// string primitive : String
// symbol primitive : Symbol
// modest BigInt : BigInt
// cheeky BigInt : BigInt
// object : Object
// array : Array
// Map : Map
// Int8Array : Int8Array