What's the difference between using instanceof and checking the constructor?

Question

Why do the following two lines return different results?

(\"test\" instanceof String) // returns false
(\"test\".constructor == String) // returns true
         


        

Answer 1

The main difference is that instanceof inspects the object's prototype chain whereas checking the constructor only checks to see if it was created from the same constructor.

Example:

function MyObject() {
    this.sayHi = function() { alert('Hi!'); }   
}

var x = new MyObject();
alert(x.constructor === Object);
alert(x instanceof Object);