Finding an object's class

To find whether an object was constructed by a certain constructor or one inheriting from it, you can use the instanceof command:

//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

Note that primitive values are not considered instances of any class:

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

Every value in JavaScript besides null and undefined also has a constructor property storing the function that was used to construct it. This even works with primitives.

//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