instanceof vs typeof

instanceof and typeof are two operators to check the type of a value.

Differences

The typeof operator checks if a value has type of primitive type which can be one of boolean, function, object, number, string, undefined and symbol (ES6).

typeof 'helloworld'; // 'string'
typeof new String('helloworld'); // 'object'

The instanceof operator checks if a value is an instance of a class or constructor function.

'helloworld' instanceof String; // false
new String('helloworld') instanceof String; // true

Good to know

If you want to check if a value is a primitive string or a String object, then you need to use both operators:

const isString = (value) => typeof value === 'string' || value instanceof String;

isString('helloworld'); // true
isString(new String('helloworld')); // true

Another approach is to rely on the toString() of Object as below:

const isString = (value) => Object.prototype.toString.call(value) === '[object String]';

isString('hello world'); // true
isString(new String('hello world')); // true
isString(10); // false

We can use similar methods to check a value against given original or wrapped primitive types:

const isBoolean = (value) => Object.prototype.toString.call(value) === '[object Boolean]';

Be careful when creating a value of primitive type with constructor. The type of value can be changed based on the way you use it. In the piece of code below, we start with creating a string from the String constructor:
```
let message = new String('hello');
message instanceof String; // true
typeof message; // 'object'
```
We are going to append the string object with another string:
```
message += ' world';
```
Now, let's look at the result of the operators:
```
message instanceof String; // false
typeof message; // 'string'
```
These type modifications are known as boxing and unboxing. Boxing is the process that wraps a primitive value by object. Unboxing extracts the wrapped primitive value from object.
There is a special case when using typeof with null:
```
typeof null; // 'object', not 'null'
```
instanceof doesn't work for primitive types.
If you want to use instanceof all the time, then you can override the behavior of instanceof by implementing a static method with the key of Symbol.hasInstance.
In the following code, we create a class called PrimitiveNumber that checks if a value is a number:
```
class PrimitiveNumber {
    static [Symbol.hasInstance](value) {
        return typeof value === 'number';
    }
}

12345 instanceof PrimitiveNumber; // true
'helloworld' instanceof PrimitiveNumber; // false
```