Welcome back to my weekly Airbnb JavaScript style guide explainer. Airbnb, the online marketplace for housing rentals, has a high quality and ‘mostly reasonable’ style guide for JavaScript, a programming language that adds dynamic behaviors to web browsers. Some of the topics in the guide are a bit advanced for beginners. This “Explain Like I’m 50” (ELI50) series will act as a plain English companion-guide to the Airbnb style guide, because good design principles shouldn’t be out of anyone’s reach.

This post will cover section 3 on objects, creating, and using them.

What the heck is an object?

Simply put an object is a way to encapsulate something in a way that can be used or modified later. Objects can have variables and functions (called methods) that act on them. The reason that objects are so useful is because you can create new objects that inherit properties from other objects. You could, for instance, make an object to represent a car which has variables to indicate it has an engine, four wheels, methods that let the user drive it. From there you can create other objects that inherit those properties from Car and add other ones like being able to drive itself. Programmers, being a lazy folk, do their best to follow something called the DRY principle (Don’t Repeat Yourself). Objects are a useful way to package code for reuse in other places.

Section 3: Objects

3.1: Use the literal syntax for object creation. eslint: no-new-object

Although the new object syntax, const myObj = new Object();, isn’t harmful it takes longer to write and can cause problems for you if you forget the new. Object literal syntax, const myObj = {};, is faster to write, easier to read, and harder to mess up. If you want to learn more about why we tend to use the second version, read the answers on this stack overflow question.

3.2: Use computed property names when creating objects with dynamic property names.

JavaScript has a nifty feature that allows you to add or create properties for objects without knowing what the name of the property will be until the program is executed. This is helpful, for instance, when you want to create objects from data because you can create the names programmatically, instead of having to input them by hand. It’s best to add the property name when you make the object so that you can keep all the properties in the same place. To learn more about computed properties or to see examples check out this guide from Mozilla.

3.7: Do not call Object.prototype methods directly, such as hasOwnProperty, propertyIsEnumerable, and isPrototypeOf.

There are some cases when calling Object.prototype methods directly, like console.log(object.hasOwnProperty(key));, can have unintended behavior. For instance if the object was null or if the method was shadowed by a property on the object.

What the heck is shadowing?

Shadowing is a concept related to scope. If you define a global variable and then a define a new variable in a more local scope, like in a function, then when you call the variable, the local variable will be the one that gets used. In this instance it could look like this:

myObject = {
  magic: true;
  hasOwnProperty: false;
myObject.hasOwnProperty(magic)                        #=> false
Object.prototype.hasOwnProperty.call(myObject, magic) #=> true

Why Should I Care?

All JavaScript values except primitives are objects. You’ll be working with objects all throughout your JavaScript career and it’s important to know how to work with them. To learn more about JavaScript objects read this guide from w3schools.

Next Up: Arrays

Next Saturday I’ll explain what Arrays are and how to use them.

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