JavaScript instanceof vs typeof

I’ve been writing JavaScript for quite some time, but only recently came across instanceof, typeof on the otherhand I’ve always been aware of.

Each serve different purposes, and have slight differences.I recently got bitten, so here’s a note to self.

Using typeof, you get a string representation of the object type. Using instanceof, you are comparing the type, specifically if the property of a constructor is in the objects prototype chain.

Here are some exaxmples of the different behaviours.

  //objects
  var Person = function() {}

  var gary = new Person();

  console.log(typeof(gary)); // "object"
  gary instanceof Person // true

  //literals
  var str = 'hello world';
  console.log(typeof(str)); // "string"
  str instanceof String // False

  var str1 = new String('hello world');
  console.log(typeof(str1)) // "object"
  str1 instanceof String // true

So in JavaScript, given that strings can be literals or objects, the correct way to test for a string would be to test against both cases.

  function isString(str) {
    return typeof(str) == 'string' || str instanceof String;
  }