Why does instanceof return false for some literals?

Primitives are a different kind of type than objects created from within Javascript. From the Mozilla API docs:

var color1 = new String("green");
color1 instanceof String; // returns true
var color2 = "coral";
color2 instanceof String; // returns false (color2 is not a String object)

I can't find any way to construct primitive types with code, perhaps it's not possible. This is probably why people use typeof "foo" === "string" instead of instanceof.

An easy way to remember things like this is asking yourself "I wonder what would be sane and easy to learn"? Whatever the answer is, Javascript does the other thing.

You can use constructor property:

'foo'.constructor == String // returns true
true.constructor == Boolean // returns true

This is defined in the ECMAScript specification Section 7.3.19 Step 3: If Type(O) is not Object, return false.

In other word, if the Obj in Obj instanceof Callable is not an object, the instanceof will short-circuit to false directly.

The primitive wrapper types are reference types that are automatically created behind the scenes whenever strings, num­bers, or Booleans are read.For example :

var name = "foo";
var firstChar = name.charAt(0);
console.log(firstChar);

This is what happens behind the scenes:

// what the JavaScript engine does
var name = "foo";
var temp = new String(name);
var firstChar = temp.charAt(0);
temp = null;
console.log(firstChar);

Because the second line uses a string (a primitive) like an object, the JavaScript engine creates an instance of String so that charAt(0) will work.The String object exists only for one statement before it’s destroyed check this

The instanceof operator returns false because a temporary object is created only when a value is read. Because instanceof doesn’t actually read anything, no temporary objects are created, and it tells us the ­values aren’t instances of primitive wrapper types. You can create primitive wrapper types manually

Or you can just make your own function like so:

function isInstanceOf(obj, clazz){
  return (obj instanceof eval("("+clazz+")")) || (typeof obj == clazz.toLowerCase());
};

usage:

isInstanceOf('','String');
isInstanceOf(new String(), 'String');

These should both return true.