Learning JavaScript, 3rd Edition

Chapter 20. Object Property Configuration and Proxies

Accessor Properties: Getters and Setters

There are two types of object properties: data properties and accessor properties. We’ve already seen both, but the accessor properties have been hidden behind some ES6 syntactic sugar (we called them “dynamic properties” in Chapter 9).

We’re familiar with function properties (or methods); accessor properties are similar except they have two functions—a getter and a setter—and when accessed, they act more like a data property than a function.

Let’s review dynamic properties. Imagine you have a User class, with methods setEmail and getEmail. We opted to use a “get” and “set” method instead of just having a property called email because we want to prevent a user from getting an invalid email address. Our class is very simple (for simplicity, we’ll treat any string with an at sign as a valid email address):

const USER_EMAIL = Symbol();
class User {
    setEmail(value) {
        if(!/@/.test(value)) throw new Error(`invalid email: ${value}`);
        this[USER_EMAIL] = value;
    }
    getEmail() {
        return this[USER_EMAIL];
    }
}

In this example, the only thing that’s compelling us to use two methods (instead of a property) is to prevent the USER_EMAIL property from receiving an invalid email address. We’re using a symbol property here to discourage accidental direct access of the property (if we used a string property called email or even _email, it would be easy to carelessly access it ...