The obj.hasOwnProperty(prop) method in JavaScript is used to determine whether a given object has a property as its own (not inherited) property. This method returns a boolean value: true if the object has the specified property as its own property, and false otherwise.

Syntax:

obj.hasOwnProperty(prop);

Parameters:

• prop: A string (or symbol) representing the name of the property to test for.

Return Value:

• true: If the object has the specified property as its own property.
• false: If the property does not exist on the object or is inherited from its prototype chain.

Key Features:

• Own Properties vs. Inherited Properties: The hasOwnProperty method distinguishes between properties that are directly defined on the object and those that are inherited from the object's prototype.
• Non-Enumerable Properties: The method can also be used to check for non-enumerable properties, which may not appear in loops like for...in.
• Symbol Properties: It works with property names that are symbols, as well as strings.

Example 1: Basic Usage

const obj = {
    name: 'Alice',
    age: 30
};

console.log(obj.hasOwnProperty('name')); // Output: true
console.log(obj.hasOwnProperty('age'));  // Output: true
console.log(obj.hasOwnProperty('gender')); // Output: false

In this example, hasOwnProperty checks for the existence of the name and age properties, returning true, while it returns false for gender since it is not defined on the object.

Example 2: Inherited Properties

const person = {
    name: 'Bob'
};

const employee = Object.create(person);
employee.position = 'Developer';

console.log(employee.hasOwnProperty('position')); // Output: true
console.log(employee.hasOwnProperty('name'));     // Output: false

Here, employee has its own property position, so hasOwnProperty('position') returns true. However, name is inherited from person, so hasOwnProperty('name') returns false.

Example 3: Checking Non-Enumerable Properties

const obj = Object.create({}, {
    prop: {
        value: 42,
        enumerable: false
    }
});

console.log(obj.hasOwnProperty('prop')); // Output: true
console.log(obj.prop); // Output: 42

for (let key in obj) {
    console.log(key); // No output because 'prop' is non-enumerable
}

In this example, prop is defined as a non-enumerable property. While hasOwnProperty returns true, it does not appear in the for...in loop.

Example 4: Using with Symbols

const sym = Symbol('unique');
const obj = {
    [sym]: 'symbol value'
};

console.log(obj.hasOwnProperty(sym)); // Output: true

This example demonstrates that hasOwnProperty can also be used with symbol properties, returning true when checking for the presence of the symbol.

Summary:

• obj.hasOwnProperty(prop) is a method that checks whether the specified property exists as an own property of the object.
• It distinguishes between properties defined directly on the object and those inherited from its prototype chain.
• This method is particularly useful in situations where you want to ensure that a property belongs directly to the object, avoiding unexpected results from inherited properties.