Any object in JavaScript is a collection of key-value pairs. The key, also known as a property, is a unique string that maps to a value which may be a Boolean, String or another object.

Let’s take a simple person object that contains properties like name, age and the employment status.

const person = {
  name: 'John',
  age: 21,
  gender: 'Male',
  employed: false,
};

• Check if a property (or key) exists in an object

console.log('country' in person); // returns false
console.log('employed' in person); // returns true
console.log(person.hasOwnProperty('gender'));

• Iterate over an object and print the key-value pairs

Object.keys(person).forEach((key) => {
  console.log(`${key}: ${person[key]}`);
});

Object.entries(person).forEach(([key, value]) => {
  console.log(`${key}: ${value}`);
});

• Prevent new properties from being added to the object

Object.preventExtensions(person);
person.full_name = 'John Q Public';
console.log(person); // the full name property is not added

• Check if new properties can be added to an object

Object.isExtensible(person);
delete person.name; // you can still delete properties

• Prevent properties from getting added or deleted

Object.seal(person);
delete person.age;
console.log(person.age); // the property is not deleted

• Check if properties can be added or deleted from any object

Object.isSealed(person);

• Prevent properties from getting added, deleted or modified

Object.freeze(person);

• Check if an object can be modified

Object.isFrozen(person);

• Combine two objects (use default values)

const defaultPerson = {
  name: 'Unknown',
  country: 'Unknown',
};

const newPerson = {
  name: 'John',
  age: 21,
};

const mergedPerson = Object.assign(defaultPerson, newPerson);
console.log(mergedPerson);

• Create a shallow clone of an object

const clone = Object.assign({}, person);
// changes to the clone will not modify the original object