Let's consider a JS object,

const author = {
  name: 'Param',
  age: 31
};

Now, let's check whether a key name exists in the object or not using the in operator.

if ('name' in author) {
  console.log('`name` key found in author object');
}

if (!('website' in author)) {
  console.log('`website` key not found in author object');
}

There is another object method hasOwnProperty to check whether key present,

console.log(author.hasOwnProperty('name')); // true
console.log(author.hasOwnProperty('website')); // false

The only difference between in and hasOwnProperty are,

• hasOwnProperty works only on the user-defined object properties. It won't work on the inherited properties from parent objects.
• in works for all keys whether it is defined or inherited from parent objects

This technique only works for the direct keys, it won't work for nested keys.

We will see a separate example to check whether the object key exists in any level of the nested object.