myVar.constructor --or— typeof myVar ---What is the difference?

Question

What is the difference between...

if(myVar.constructor == String)

and

if(typeof myVar == \"string\")

Answer 1

The expression myVar.constructor just returns the value of the member "constructor". It's completely possible and legal for a given object to have a constructor which points to string (or anything else). For example

function Apple() {}
var x = new Apple();
x.constructor = String
if (x.constructor == String) {
  // True
}

Using the typeof operator though provides the information you're looking for

if (typeof myVar == 'string') {
  // It's a string
}

Answer 2

typeof can be a little misleading. Note the following behavior:

typeof fails on real String instances:

typeof "snub" 
  // "string" -- as expected

typeof new String("snub") 
  // "object" -- even if it's still the string you're after

---

.constructor consistently yields the expected result:

"snub".constructor == String 
  // true -- as expected

new String("snub").constructor == String 
  // true -- as expected

All primitive values in JavaScript have a corresponding object wrapper. It's from where primitives find a prototype chain to inherit from.

Values in JavaScript can be expressed in primitive form (specially inheriting from their respective objects), or directly in object form. It's possible some of your strings are converted into String instances along the way -- by a library perhaps -- and you won't even know it. String objects and primitives generally behave the same way. Except for with typeof, of course.

Directly in object form -- even while the inner value may represent something that is a string, number, or otherwise -- typeof will respond with "object".

.constructor is a more reliable way to identify an object's type, and is directly available to your primitive values (like all the rest of their inherited functionality) thanks to a language feature called Autoboxing.