When do I need to use hasOwnProperty()

do not access object.prototype method 'hasownproperty' from target object
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I read that we should always use hasOwnProperty when looping an object, because the object can be modified by something else to include some keys we don't want

But is this always required? Are there situations where it's not needed? Is this required for local variables too?

function my(){
  var obj = { ... };
  for(var key in obj){



I just don't like adding an extra if inside the loop if I don't have to.


This guy says I shouldn't use it all anymore.

Object.hasOwnProperty determines if the whole property is defined in the object itself or in the prototype chain.

In other words: do the so-called check if you want properties (either with data or functions) coming from no other place than the object itself.

For example:

function A() {
   this.x = "I'm an own property";

A.prototype.y = "I'm not an own property";

var instance = new A();
var xIsOwnProperty = instance.hasOwnProperty("x"); // true
var yIsOwnProperty = instance.hasOwnProperty("y"); // false
Do you want to avoid the whole check if you want own properties only?

Since ECMA-Script 5.x, Object has a new function Object.keys which returns an array of string where its items are the own properties from a given object:

var instance = new A();
// This won't contain "y" since it's in the prototype so
// it's not an "own object property"
var ownPropertyNames = Object.keys(instance);

Also, since ECMA-Script 5.x, Array.prototype has Array.prototype.forEach which lets perform a for-each loop fluently:

Object.keys(instance).forEach(function(ownPropertyName) {
    // This function will be called for each found "own property", and
    // you don't need to do the instance.hasOwnProperty check anymore

Object.prototype.hasOwnProperty(), The in operator will tell you whether an object (or array) has a property name which function litList () {} litList.prototype. Usually we only want to check for properties that belong to the object itself, not its prototype chain. Because in JavaScript arrays also inherit from Object , they can use hasOwnProperty as well, though  This method can be used to determine whether an object has the specified property as a direct property of that object; unlike the in operator, this method does not check for a property in the object's prototype chain. If an Object is an Array, hasOwnProperty method can check whether an index exists.

When you're using for (var key in obj) it will loop through the given object + its parent objects' properties on the prototype chain until it reaches the end of the chain. As you want to check only specific object's properties, you need to use hasOwnProperty.

This is not needed in for (var i = 0; i < length; i++) or data.forEach()

The Uses of 'in' vs 'hasOwnProperty', A good rule of thumb is that if you're looking to see whether an object has a property, you should use hasOwnProperty() . If you're looking to see if  Using hasOwnProperty () to check if the property belongs to the object exist or not: The main benefit of JavaScript hasOwnProperty () method is it can be initialized with any object by just using the string as an argument. It quickly returns true if the value is available to the object, else it returns false.

hasOwnProperty expects the property name as a string.

When you call Test.hasOwnProperty(name) you are passing it the value of the name variable (which doesn't exist), just as it would if you wrote alert(name).

Every object descended from Object inherits the hasOwnPropertymethod. This method can be used to determine whether an object has the specified property as a direct property of that object;


The Difference Between in and hasOwnProperty in JavaScript , Here is how I want to use it. var content = "The author name is "+ myObj;. For it to properly work, I would have to change the function toText to  console.log(hollow.hasOwnProperty(‘id’)); // “Uncaught TypeError: hollow.hasOwnProperty is not a function” In the snippet above, we initialised a new object using Object.create(null). Because null was passed in, the result is an object that is void of the default object’s prototype chain. No prototype means no hasOwnProperty method.

When to use Object.hasOwnProperty - JavaScript, We use cookies to ensure you have the best browsing experience on our website​. The hasOwnProperty() method in JavaScript is used to check whether the browsers supported by JavaScript hasOwnProperty() method are listed below:. Because of this, using in to detect whether an object possesses a given property can be a bit deceptive. Usually we only want to check for properties that belong to the object itself, not its prototype chain. Fortunately, JavaScript has a solution for that. It is called hasOwnProperty.

JavaScript, Most objects in JavaScript have access (via inheritance from Object.prototype ) See the somewhat related "The body of a for in should be wrapped in an if The solution to this error is to simply not use hasOwnProperty as a property identifer. Consider an object initialised using Object.create(null): const hollow = Object.create(null); hollow.id = 99; console.log(hollow.hasOwnProperty('id')); // "Uncaught TypeError: hollow.hasOwnProperty is not a function" In the snippet above, we initialised a new object using Object.create(null).

'hasOwnProperty' is a really bad name, hasOwnProperty() — check whether a property is inherited false if object does not have a property with the specified name or if it inherits that property from its  Both my in and === examples will do a scope chain lookup if the property is not found on the object itself. This could lead to some, shall we say, unexpected results. The hasOwnProperty method might be a safer bet in most cases, but even that can "lie" in some cases.

  • @elad.chen Thanks ;P Simple but still efficient!
  • I would consider adding the usage of Object.keys(obj).forEach(..); As this will prevent prototype lookups..
  • @elad.chen Right, I can add this. My concern is that a simple Q&A can turn into an entire Wikipedia page haha
  • I see. Anyway if someone sees my comment, he'll have a basic idea of what I mean.
  • @elad.chen I've added the thing about Object.keys. See my update. I was talking about adding even more stuff... This answer can turn into a bible about prototype chaining, object creation approaches..... you know :D