Why a number isn’t instance of Number with typeof and instanceof? Code Answer

I need to take all elements of given type from an array, like this [{ a: 'a' }, 4, /a-z/, 6, [1, 2, 3], i=>i], and when I provide type Number as argument to ofType(type) it returns [ ] :

Array.prototype.ofType = function (type) {
    let newArr = []; 
    this.forEach(i => i instanceof type ? newArr.push(i) : newArr );
    return newArr
}

in other cases , like [{ a: 'a' }, 4, /a-z/, 6, [1, 2, 3], i=>i].ofType(RegExp) it works correctly.

Answer

JavaScript has number primitives and Number objects. A number primitive isn’t instanceof anything because it isn’t an object, and only objects are “instances” of something (“instance” being a term of art from object oriented programming); so instanceof is not useful for primitives.

That’s where typeof comes in. The result of typeof for primitive numbers is "number".

Since instanceof expects a constructor function as the right hand operand, you could have your ofType function branch based on whether it gets a function or a string, using instanceof if it’s a function and typeof if it’s a string. Using your forEach loop:

Array.prototype.ofType = function (type) {
    let newArr = []; 
    if (typeof type === "string") {
        this.forEach(i => typeof i === type ? newArr.push(i) : newArr );
    } else {
        this.forEach(i => i instanceof type ? newArr.push(i) : newArr );
    }
    return newArr;
};

But I wouldn’t use forEach for this, I’d use filter:

Array.prototype.ofType = function (type) {
    let newArr;
    if (typeof type === "string") {
        newArr = this.filter(i => typeof i === type);
    } else {
        newArr = this.filter(i => i instanceof type);
    }
    return newArr;
};

Also, I strongly recommend not adding enumerable properties to Array.prototype, it breaks code that incorrectly uses for-in loops on arrays. Use Object.defineProperty instead:

Object.defineProperty(Array.prototype, "ofType", {
    value(type) {
        let newArr;
        if (typeof type === "string") {
            newArr = this.filter(i => typeof i === type);
        } else {
            newArr = this.filter(i => i instanceof type);
        }
        return newArr;
    },
    writable: true,
    configurable: true,
    enumerable: false // This is the default, so you could leave it off
});

Live Example:

Object.defineProperty(Array.prototype, "ofType", {
    value(type) {
        let newArr;
        if (typeof type === "string") {
            newArr = this.filter(i => typeof i === type);
        } else {
            newArr = this.filter(i => i instanceof type);
        }
        return newArr;
    },
    writable: true,
    configurable: true,
    enumerable: false // This is the default, so you could leave it off
});

console.log([1, "2", 3, "4"].ofType("number"));

console.log([new Date, "2", new Date, "4"].ofType(Date));