Why does instanceof return false for some literals?

Question

"foo" instanceof String //=> false
"foo" instanceof Object //=> false

true instanceof Boolean //=> false
true instanceof Object //=>         


        

Answer 1

You can use constructor property:

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

Answer 2

 typeof(text) === 'string' || text instanceof String; 

you can use this, it will work for both case as

1. var text="foo"; // typeof will work

2. String text= new String("foo"); // instanceof will work

Answer 3

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.

Answer 4

For me the confusion caused by

"str".__proto__ // #1
=> String

So "str" istanceof String should return true because how istanceof works as below:

"str".__proto__ == String.prototype // #2
=> true

Results of expression #1 and #2 conflict each other, so there should be one of them wrong.

#1 is wrong

I figure out that it caused by the __proto__ is non standard property, so use the standard one:Object.getPrototypeOf

Object.getPrototypeOf("str") // #3
=> TypeError: Object.getPrototypeOf called on non-object

Now there's no confusion between expression #2 and #3