How to Check if an Object has a Specific Property in JavaScript ?

In JavaScript, objects can have properties that store data or functions. Sometimes it is necessary to check whether an object has a specific property. This can be useful, for example, when you want to avoid calling a function on an object that doesn't have that function defined as a property. In this article, we will be discussing different approaches to check whether an object has a specific property or not.

These are the following approaches:

Using the "in" operator

The "in" operator checks if a property exists in an object or its prototype chain.

Syntax:

if ('propertyName' in objectName) {
       // Code to execute if property exists
}

Here, "propertyName" is the name of the property you want to check, and "objectName" is the name of the object you want to check.

Example: This example shows the use of in operator.

const person = {
    name: 'John',
    age: 30,
    address: {
        city: 'New York',
        state: 'NY'
    }
};

if ('age' in person) {
    console.log('Person object has the age property');
}

if ('email' in person) {
    console.log('Person object has the email property');
} else {
    console.log('Person object does not have the email property');
}

Person object has the age property
Person object does not have the email property

Using hasOwnProperty() method

The hasOwnProperty() method checks if an object has a property of its own and is not inherited from its prototype chain.

Syntax:

if (objectName.hasOwnProperty('propertyName')) {
       // Code to execute if property exists
}

Here, "propertyName" is the name of the property you want to check, and "objectName" is the name of the object you want to check.

Example: This example shows the use of hasOwnProperty() method.

const person = {
    name: 'John',
    age: 30,
    address: {
        city: 'New York',
        state: 'NY'
    }
};

if (person.hasOwnProperty('age')) {
    console.log('Person object has the age property');
}

if (person.hasOwnProperty('email')) {
    console.log('Person object has the email property');
} else {
    console.log('Person object does not have the email property');
}

Person object has the age property
Person object does not have the email property

Using the "typeof" Operator

You can also use the "typeof" operator to check if an object has a specific property. 

Syntax:

if (typeof object.property !== 'undefined') {
      // Property exists in object
} else {
      // Property does not exist in object
}

Here, "property" is the name of the property that you want to check for, and "object" is the object that you want to check. The "typeof" operator returns the type of the value of the specified property, so if the property does not exist in the object, it will return "undefined".

Example: In this example, we're using the "typeof" operator to determine the type of the "myVar" variable, which is a string. You can use the "typeof" operator to check for different types of values, such as "number", "boolean", "undefined", "object", "function", and "symbol".

const myVar = "Hello World";

console.log(typeof myVar); // Output: string

string