instanceof vs typeof

Differences

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

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

Good to know

1. 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]';

2. 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'

   This 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.
3. There is a special when using typeof with null:

   typeof null;    // 'object', not 'null'

4. 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