JavaScript Objects Quick Reference

November 26, 2020

Everything you need to know for working with JavaScript Objects

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

Javascript

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Amit Agarwal

Google Developer Expert, Google Cloud Champion

Amit Agarwal is a Google Developer Expert in Google Workspace and Google Apps Script. He holds an engineering degree in Computer Science (I.I.T.) and is the first professional blogger in India.

Amit has developed several popular Google add-ons including Mail Merge for Gmail and Document Studio. Read more on Lifehacker and YourStory