Why does instanceof return false for some literals?

Question

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

// the tests against Object really don't make sense

Array literals and Object literals match...

[0,1] instanceof Array //=> true
{0:1} instanceof Object //=> true

Why don't all of them? Or, why don't they all not?
And, what are they an instance of, then?

It's the same in FF3, IE7, Opera, and Chrome. So, at least it's consistent.

---

Missed a few.

12.21 instanceof Number //=> false
/foo/ instanceof RegExp //=> true

Answer 1

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.

Answer 2

I use:

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

Because in JavaScript strings can be literals or objects.

Answer 3

In JavaScript everything is an object (or may at least be treated as an object), except primitives (booleans, null, numbers, strings and the value undefined (and symbol in ES6)):

console.log(typeof true);           // boolean
console.log(typeof 0);              // number
console.log(typeof "");             // string
console.log(typeof undefined);      // undefined
console.log(typeof null);           // object
console.log(typeof []);             // object
console.log(typeof {});             // object
console.log(typeof function () {}); // function

As you can see objects, arrays and the value null are all considered objects (null is a reference to an object which doesn't exist). Functions are distinguished because they are a special type of callable objects. However they are still objects.

On the other hand the literals true, 0, "" and undefined are not objects. They are primitive values in JavaScript. However booleans, numbers and strings also have constructors Boolean, Number and String respectively which wrap their respective primitives to provide added functionality:

console.log(typeof new Boolean(true)); // object
console.log(typeof new Number(0));     // object
console.log(typeof new String(""));    // object

As you can see when primitive values are wrapped within the Boolean, Number and String constructors respectively they become objects. The instanceof operator only works for objects (which is why it returns false for primitive values):

console.log(true instanceof Boolean);              // false
console.log(0 instanceof Number);                  // false
console.log("" instanceof String);                 // false
console.log(new Boolean(true) instanceof Boolean); // true
console.log(new Number(0) instanceof Number);      // true
console.log(new String("") instanceof String);     // true

As you can see both typeof and instanceof are insufficient to test whether a value is a boolean, a number or a string - typeof only works for primitive booleans, numbers and strings; and instanceof doesn't work for primitive booleans, numbers and strings.

Fortunately there's a simple solution to this problem. The default implementation of toString (i.e. as it's natively defined on Object.prototype.toString) returns the internal [[Class]] property of both primitive values and objects:

function classOf(value) {
    return Object.prototype.toString.call(value);
}

console.log(classOf(true));              // [object Boolean]
console.log(classOf(0));                 // [object Number]
console.log(classOf(""));                // [object String]
console.log(classOf(new Boolean(true))); // [object Boolean]
console.log(classOf(new Number(0)));     // [object Number]
console.log(classOf(new String("")));    // [object String]

The internal [[Class]] property of a value is much more useful than the typeof the value. We can use Object.prototype.toString to create our own (more useful) version of the typeof operator as follows:

function typeOf(value) {
    return Object.prototype.toString.call(value).slice(8, -1);
}

console.log(typeOf(true));              // Boolean
console.log(typeOf(0));                 // Number
console.log(typeOf(""));                // String
console.log(typeOf(new Boolean(true))); // Boolean
console.log(typeOf(new Number(0)));     // Number
console.log(typeOf(new String("")));    // String

Hope this article helped. To know more about the differences between primitives and wrapped objects read the following blog post: The Secret Life of JavaScript Primitives

Answer 4

You can use constructor property:

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

Answer 5

 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 6

I believe I have come up with a viable solution:

Object.getPrototypeOf('test') === String.prototype    //true
Object.getPrototypeOf(1) === String.prototype         //false

Answer 7

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 8

https://www.npmjs.com/package/typeof

Returns a string-representation of instanceof (the constructors name)

function instanceOf(object) {
  var type = typeof object

  if (type === 'undefined') {
    return 'undefined'
  }

  if (object) {
    type = object.constructor.name
  } else if (type === 'object') {
    type = Object.prototype.toString.call(object).slice(8, -1)
  }

  return type.toLowerCase()
}

instanceOf(false)                  // "boolean"
instanceOf(new Promise(() => {}))  // "promise"
instanceOf(null)                   // "null"
instanceOf(undefined)              // "undefined"
instanceOf(1)                      // "number"
instanceOf(() => {})               // "function"
instanceOf([])                     // "array"

Answer 9

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

Answer 10

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.