Finding an objects class

// 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