JavaScript Objects Quick Reference

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Everything you need to know for working with JavaScript Objects

Published in: JavaScript

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
Published in: JavaScript

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Amit Agarwal

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. He is the developer of Mail Merge for Gmail and Document Studio. Read more on Lifehacker and YourStory

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