JavaScript Object.hasOwnProperty()

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Object.hasOwnProperty() is used in JavaScript to check if an object has a specific property as its own—not something inherited from another object.

Key takeaway

hasOwnProperty() is useful for distinguishing between properties that belong to the object and those that come from its prototype or inheritance.

Example

let person = {
  name: 'John',
  age: 30
};

console.log(person.hasOwnProperty('name')); // true
console.log(person.hasOwnProperty('gender')); // false
  • What it does: It checks if the person object has a property called ‘name’ or ‘gender.’
  • True result: If the property exists directly in the object (like ‘name’ in this example), it returns true.
  • False result: If the property doesn’t exist or is inherited, it returns false.

It’s a way to confirm if a property really belongs to the object you’re working with, not something from its prototype or parent objects.

Here are a few more examples to help clarify how Object.hasOwnProperty() works:

Example 1: Direct property check

let book = {
  title: 'The Great Gatsby',
  author: 'F. Scott Fitzgerald'
};

console.log(book.hasOwnProperty('title')); // true
console.log(book.hasOwnProperty('publisher')); // false
  • The book object has a title property, so it returns true.
  • It doesn’t have a publisher property, so it returns false.

Example 2: Checking inherited properties

function Animal(name) {
  this.name = name;
}

Animal.prototype.species = 'Mammal';

let dog = new Animal('Buddy');

console.log(dog.hasOwnProperty('name')); // true
console.log(dog.hasOwnProperty('species')); // false
  • The dog object has its own name property, so it returns true.
  • species is inherited from the prototype (Animal.prototype), so hasOwnProperty() returns false.

Example 3: Using in loops with hasOwnProperty()

Sometimes when you loop through object properties, you might want to filter out inherited ones. Here’s how hasOwnProperty() helps:

let car = {
  make: 'Honda',
  model: 'Civic'
};

for (let prop in car) {
  if (car.hasOwnProperty(prop)) {
    console.log(prop + ': ' + car[prop]);
  }
}
  • This loop prints only the properties that belong directly to the car object (make and model).

Example 4: Using with inherited methods

let person = {
  name: 'Alice'
};

console.log(person.hasOwnProperty('toString')); // false
console.log(person.hasOwnProperty('name')); // true
  • toString is an inherited method, so hasOwnProperty() returns false.
  • name is a direct property of person, so it returns true.

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