Instanceof inconsistency

Question

Could someone please explain the following?

[] instanceof Array; // true
'' instanceof String; // false

Answer 1

Note the following:

"" instanceof String;             // => false
new String("") instanceof String; // => true

instanceof requires an object, but "" is a string literal, and not a String object. Note the following types using the typeof function:

typeof ""             // => "string"
typeof new String("") // => "object"
typeof []             // => "object"
typeof new Array()    // => "object"

Answer 2

It's because '' is primitive, not an object.

Some primitives in JavaScript can have an object wrapper. These are created when you make an instance of the wrapper using the built in constructor with new.

The new is typically necessary because often times the function will coerce to a primitive if you exclude new.

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

The primitives that have Object wrappers are string, number and boolean.

The primitives that do not are null and undefined.