Chapter 6 Cheatsheet: Prototypes

This cheatsheet summarizes the mechanics of Prototypal Inheritance in JavaScript covered in Chapter 6.

While the class keyword is the modern standard for structuring object-oriented code, it is vital to understand that JavaScript relies entirely on prototypes under the hood.

Prototypal Inheritance

Instead of classes creating instances, objects inherit directly from other objects.

Use Object.create(parentObj) to create a new, empty object that uses parentObj as its prototype fallback.

const baseUser = { type: "User", canLogin: true };

// Creates a new object linked to baseUser
const adminUser = Object.create(baseUser);
adminUser.type = "Admin"; // Overrides the inherited property

console.log(adminUser.canLogin); // true (Inherited)
console.log(adminUser.type);     // "Admin" (Overridden)

The Prototype Chain

When you attempt to access a property on an object (e.g., obj.name), JavaScript performs the following lookup:

1. Does obj possess name directly?
2. If no, does obj’s prototype possess name?
3. If no, does that prototype’s prototype possess name?
4. This continues until it finds the property or hits the root null (returning undefined).

Inspecting Prototypes

You can mathematically verify inheritance and inspect chains using built-in Object methods.

// Get the direct parent of an object
const parent = Object.getPrototypeOf(childObj);