typeof "String" or
typeof Date(2011,01,01)

typeof 42

"number"

typeof true (valid values true and false)

"boolean"

typeof {} or
typeof [] or
typeof null or
typeof /aaa/ or
typeof Error()

typeof function(){}

var var1; typeof var1

//We want this function to take the sum of the numbers passed to it
//It can be called as sum(1, 2, 3) or sum([1, 2, 3]) and should give 6
function sum(...arguments) {
if (arguments.length === 1) {
const [firstArg] = arguments
if (firstArg instanceof Array) { //firstArg is something like [1, 2, 3]
return sum(...firstArg) //calls sum(1, 2, 3)
}
}
return arguments.reduce((a, b) => a + b)
}

console.log(sum(1, 2, 3)) //6
console.log(sum([1, 2, 3])) //6
console.log(sum(4)) //4

console.log(2 instanceof Number) //false
console.log('abc' instanceof String) //false
console.log(true instanceof Boolean) //false
console.log(Symbol() instanceof Symbol) //false

//Whereas instanceof also catches instances of subclasses,
//using obj.constructor does not
console.log([] instanceof Object, [] instanceof Array) //true true
console.log([].constructor === Object, [].constructor === Array) //false true
function isNumber(value) {
//null.constructor and undefined.constructor throw an error when accessed
if (value === null || value === undefined) return false
return value.constructor === Number
}
console.log(isNumber(null), isNumber(undefined)) //false false
console.log(isNumber('abc'), isNumber([]), isNumber(() => 1)) //false false false
console.log(isNumber(0), isNumber(Number('10.1')), isNumber(NaN)) //true true true

Object.prototype.toString.call("String")

Object.prototype.toString.call(42)

Object.prototype.toString.call(true)

Object.prototype.toString.call(Object()) or
Object.prototype.toString.call({})

Object.prototype.toString.call(function(){})

Object.prototype.toString.call(new Date(2015,10,21))

Object.prototype.toString.call(new RegExp()) or
Object.prototype.toString.call(/foo/);

Object.prototype.toString.call([]);

Object.prototype.toString.call(null);

Object.prototype.toString.call(undefined);

Object.prototype.toString.call(Error());