The instanceof
keyword in JavaScript can be quite confusing when it is first encountered, as people tend to think that JavaScript is not an object-oriented programming language.
What is it?
What problems does it solve?
When is it appropriate and when not?
"foo" instanceof String
=> false, 1 instanceof Number
=> false, {} instanceof Object
=> false. Say what?!
"foo" instanceof String => false
is correct, because typeof "foo" == 'string'
. new String("foo") instanceof String => true
, because typeof String == 'function'
- you should treat function like class (definition of class). Variable become instanceof
some function
(class) when you assign it as var v = new AnythingWhatTypeofEqualsFunction()
. The same applies to 1
. typeof 1 == 'number'
- 'number' is not 'function' :) Next - {} instanceof Object
is TRUE
in node and modern browsers
({}) instanceof Object
will return true
. In fact the code you wrote will give you an error.
instanceof
The Left Hand Side (LHS) operand is the actual object being tested to the Right Hand Side (RHS) operand which is the actual constructor of a class. The basic definition is:
Checks the current object and returns true if the object is of the specified object type.
Here are some good examples and here is an example taken directly from Mozilla's developer site:
var color1 = new String("green");
color1 instanceof String; // returns true
var color2 = "coral"; //no type specified
color2 instanceof String; // returns false (color2 is not a String object)
One thing worth mentioning is instanceof
evaluates to true if the object inherits from the class's prototype:
var p = new Person("Jon");
p instanceof Person
That is p instanceof Person
is true since p
inherits from Person.prototype
.
Per the OP's request
When you declare a variable you give it a specific type.
For instance:
int i;
float f;
Customer c;
The above show you some variables, namely i
, f
, and c
. The types are integer
, float
and a user defined Customer
data type. Types such as the above could be for any language, not just JavaScript. However, with JavaScript when you declare a variable you don't explicitly define a type, var x
, x could be a number / string / a user defined data type. So what instanceof
does is it checks the object to see if it is of the type specified so from above taking the Customer
object we could do:
var c = new Customer();
c instanceof Customer; //Returns true as c is just a customer
c instanceof String; //Returns false as c is not a string, it's a customer silly!
Above we've seen that c
was declared with the type Customer
. We've new'd it and checked whether it is of type Customer
or not. Sure is, it returns true. Then still using the Customer
object we check if it is a String
. Nope, definitely not a String
we newed a Customer
object not a String
object. In this case, it returns false.
It really is that simple!
There's an important facet to instanceof that does not seem to be covered in any of the comments thus far: inheritance. A variable being evaluated by use of instanceof could return true for multiple "types" due to prototypal inheritance.
For example, let's define a type and a subtype:
function Foo(){ //a Foo constructor
//assign some props
return this;
}
function SubFoo(){ //a SubFoo constructor
Foo.call( this ); //inherit static props
//assign some new props
return this;
}
SubFoo.prototype = Object.create(Foo.prototype); // Inherit prototype
SubFoo.prototype.constructor = SubFoo;
Now that we have a couple of "classes" lets make some instances, and find out what they're instances of:
var
foo = new Foo()
, subfoo = new SubFoo()
;
alert(
"Q: Is foo an instance of Foo? "
+ "A: " + ( foo instanceof Foo )
); // -> true
alert(
"Q: Is foo an instance of SubFoo? "
+ "A: " + ( foo instanceof SubFoo )
); // -> false
alert(
"Q: Is subfoo an instance of Foo? "
+ "A: " + ( subfoo instanceof Foo )
); // -> true
alert(
"Q: Is subfoo an instance of SubFoo? "
+ "A: " + ( subfoo instanceof SubFoo )
); // -> true
alert(
"Q: Is subfoo an instance of Object? "
+ "A: " + ( subfoo instanceof Object )
); // -> true
See that last line? All "new" calls to a function return an object that inherits from Object. This holds true even when using object creation shorthand:
alert(
"Q: Is {} an instance of Object? "
+ "A: " + ( {} instanceof Object )
); // -> true
And what about the "class" definitions themselves? What are they instances of?
alert(
"Q: Is Foo an instance of Object? "
+ "A:" + ( Foo instanceof Object)
); // -> true
alert(
"Q: Is Foo an instance of Function? "
+ "A:" + ( Foo instanceof Function)
); // -> true
I feel that understanding that any object can be an instance of MULTIPLE types is important, since you my (incorrectly) assume that you could differentiate between, say and object and a function by use of instanceof
. As this last example clearly shows a function is an object.
This is also important if you are using any inheritance patterns and want to confirm the progeny of an object by methods other than duck-typing.
Hope that helps anyone exploring instanceof
.
SubFoo.prototype = new Foo();
you can add more methods to it, and the subfoo instanceof Foo
check will still pass as well as subfoo instanceof SubFoo
The other answers here are correct, but they don't get into how instanceof
actually works, which may be of interest to some language lawyers out there.
Every object in JavaScript has a prototype, accessible through the __proto__
property. Functions also have a prototype
property, which is the initial __proto__
for any objects created by them. When a function is created, it is given a unique object for prototype
. The instanceof
operator uses this uniqueness to give you an answer. Here's what instanceof
might look like if you wrote it as a function.
function instance_of(V, F) {
var O = F.prototype;
V = V.__proto__;
while (true) {
if (V === null)
return false;
if (O === V)
return true;
V = V.__proto__;
}
}
This is basically paraphrasing ECMA-262 edition 5.1 (also known as ES5), section 15.3.5.3.
Note that you can reassign any object to a function's prototype
property, and you can reassign an object's __proto__
property after it is constructed. This will give you some interesting results:
function F() { }
function G() { }
var p = {};
F.prototype = p;
G.prototype = p;
var f = new F();
var g = new G();
f instanceof F; // returns true
f instanceof G; // returns true
g instanceof F; // returns true
g instanceof G; // returns true
F.prototype = {};
f instanceof F; // returns false
g.__proto__ = {};
g instanceof G; // returns false
__proto__
" property is not allowed in IE. If I recall correctly, direct manipulation of the property is not included in the ECMA spec either so it's probably a bad idea to use it for assignment other than in academic pursuits.
Object.getPrototypeOf(o)
, this will be the same as the __proto__
you describe, but conforms to ECMAScript.
I think it's worth noting that instanceof is defined by the use of the "new" keyword when declaring the object. In the example from JonH;
var color1 = new String("green");
color1 instanceof String; // returns true
var color2 = "coral";
color2 instanceof String; // returns false (color2 is not a String object)
What he didn't mention is this;
var color1 = String("green");
color1 instanceof String; // returns false
Specifying "new" actually copied the end state of the String constructor function into the color1 var, rather than just setting it to the return value. I think this better shows what the new keyword does;
function Test(name){
this.test = function(){
return 'This will only work through the "new" keyword.';
}
return name;
}
var test = new Test('test');
test.test(); // returns 'This will only work through the "new" keyword.'
test // returns the instance object of the Test() function.
var test = Test('test');
test.test(); // throws TypeError: Object #<Test> has no method 'test'
test // returns 'test'
Using "new" assigns the value of "this" inside the function to the declared var, while not using it assigns the return value instead.
new
with any of JavaScript's types which makes the accepted answer much more confusing to beginners. text = String('test')
and options = {}
aren't going to tested by instanceof
but, rather, with typeof
instead.
And you can use it for error handling and debugging, like this:
try{
somefunction();
}
catch(error){
if (error instanceof TypeError) {
// Handle type Error
} else if (error instanceof ReferenceError) {
// Handle ReferenceError
} else {
// Handle all other error types
}
}
What is it?
Javascript is a prototypal language which means it uses prototypes for 'inheritance'. the instanceof
operator tests if a constructor function's prototype
propertype is present in the __proto__
chain of an object. This means that it will do the following (assuming that testObj is a function object):
obj instanceof testObj;
Check if prototype of the object is equal to the prototype of the constructor: obj.__proto__ === testObj.prototype >> if this is true instanceof will return true. Will climb up the prototype chain. For example: obj.__proto__.__proto__ === testObj.prototype >> if this is true instanceof will return true. Will repeat step 2 until the full prototype of object is inspected. If nowhere on the prototype chain of the object is matched with testObj.prototype then instanceof operator will return false.
Example:
function Person(name) { this.name = name; } var me = new Person('Willem'); console.log(me instanceof Person); // true // because: me.__proto__ === Person.prototype // evaluates true console.log(me instanceof Object); // true // because: me.__proto__.__proto__ === Object.prototype // evaluates true console.log(me instanceof Array); // false // because: Array is nowhere on the prototype chain
What problems does it solve?
It solved the problem of conveniently checking if an object derives from a certain prototype. For example, when a function recieves an object which can have various prototypes. Then, before using methods from the prototype chain, we can use the instanceof
operator to check whether the these methods are on the object.
Example:
function Person1 (name) { this.name = name; } function Person2 (name) { this.name = name; } Person1.prototype.talkP1 = function () { console.log('Person 1 talking'); } Person2.prototype.talkP2 = function () { console.log('Person 2 talking'); } function talk (person) { if (person instanceof Person1) { person.talkP1(); } if (person instanceof Person2) { person.talkP2(); } } const pers1 = new Person1 ('p1'); const pers2 = new Person2 ('p2'); talk(pers1); talk(pers2);
Here in the talk()
function first is checked if the prototype is located on the object. After this the appropriate method is picked to execute. Not doing this check could result in executing a method which doesn't exist and thus a reference error.
When is it appropriate and when not?
We kind of already went over this. Use it when you are in need of checking the prototype of an object before doing something with it.
PersonX.prototype.talk
, so that the talk
function could simply do person.talk()
.
__proto__
in documentation, it's deprecated - write Object.getPrototype()
instead
//Vehicle is a function. But by naming conventions
//(first letter is uppercase), it is also an object
//constructor function ("class").
function Vehicle(numWheels) {
this.numWheels = numWheels;
}
//We can create new instances and check their types.
myRoadster = new Vehicle(4);
alert(myRoadster instanceof Vehicle);
On the question "When is it appropriate and when not?", my 2 cents:
instanceof
is rarely useful in production code, but useful in tests where you want to assert that your code returns / creates objects of the correct types. By being explicit about the kinds of objects your code is returning / creating, your tests become more powerful as a tool for understanding and documenting your code.
I just found a real-world application and will use it more often now, I think.
If you use jQuery events, sometimes you want to write a more generic function which may also be called directly (without event). You can use instanceof
to check if the first parameter of your function is an instance of jQuery.Event
and react appropriately.
var myFunction = function (el) {
if (el instanceof $.Event)
// event specific code
else
// generic code
};
$('button').click(recalc); // Will execute event specific code
recalc('myParameter'); // Will execute generic code
In my case, the function needed to calculate something either for all (via click event on a button) or only one specific element. The code I used:
var recalc = function (el) {
el = (el == undefined || el instanceof $.Event) ? $('span.allItems') : $(el);
// calculate...
};
instanceof
is just syntactic sugar for isPrototypeOf
:
function Ctor() {}
var o = new Ctor();
o instanceof Ctor; // true
Ctor.prototype.isPrototypeOf(o); // true
o instanceof Ctor === Ctor.prototype.isPrototypeOf(o); // equivalent
instanceof
just depends on the prototype of a constructor of an object.
A constructor is just a normal function. Strictly speaking it is a function object, since everything is an object in Javascript. And this function object has a prototype, because every function has a prototype.
A prototype is just a normal object, which is located within the prototype chain of another object. That means being in the prototype chain of another object makes an object to a prototype:
function f() {} // ordinary function
var o = {}, // ordinary object
p;
f.prototype = o; // oops, o is a prototype now
p = new f(); // oops, f is a constructor now
o.isPrototypeOf(p); // true
p instanceof f; // true
The instanceof
operator should be avoided because it fakes classes, which do not exist in Javascript. Despite the class
keyword not in ES2015 either, since class
is again just syntactic sugar for...but that's another story.
instanceof
is derived from isPrototypeOf
. I call this syntactic sugar. And your MDN source is a joke, isn't it?
instanceof
does not do the same thing as isPrototypeOf
. That's it.
"" instanceof object
is false
, but new String('') instanceof object
is true
. Everything is not an object in Javascript
@SebastianSimon my last answer is 8 years old (when I lamed) and it is possible I wrote some bull*hit :)
Briefly - currently the only cases when I am using instanceof
is when I am using class
instances and behaviour depends on class I will receive, eg. I want to distinct if 404 is ErrorA(resource not exists) or ErrorB(service not found) - library response codes were confusing but for my luck it throwed using different error classes.
Definitely (currently) I would NOT use it to check types that reflects primitives - you cannot be sure if library is returning 'msg'
or new String('msg')
.
Both of them have methods that belongs to String
class because 'msg'
primitive is internally wrapped into string object. Internally means by interpreter. They are both Strings
but instanceof
operator seems to be insufficient here - to check if sth. is primitive or class I would use mix of typeof
&& instanceof
- but only for something returned from external JS library.
Currenlty TypeScript is solving this problem and you no longer have to use such officious checks with typeof
and instanceof
.
Success story sharing
color1 instanceof String;
this will return true because color1 is a string.person p = new person()
p is now a person type and not a string type.var zero = 0; alert(zero); zero = "0"; alert(zero)
we went from a primitiveint
to a primitivestring
without any issues.int
to primitivestring
.