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