Module pattern

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With the module design pattern we create encapsulation: the variables and functions (as methods) are kept private inside the module and cannot be overwritten. This design pattern is familiar from Node.js development: every package you import into your project and download from NPM is a module.

Generally you will create the module as a class, import it and then instantiate a new instance. However for private development, you could just as well use an object and duplicate it with Object.create.

Example

Here is an example of a simple module that returns the age of a person.

export default class Age {
  constructor(name, birthYear) {
    this.name = name;
    this.birthYear = birthYear;
  }
  currentYear() {
    return new Date().getFullYear();
  }
  get age() {
    return this.currentYear() - this.birthYear;
  }
}
const martha = new Age("Martha", 1997);
console.log(martha.age); // 24

Controlling access

In the example above, aValue could be edited in instantiations of the class. Given that modules should not be overwrittable, you could make it a private property on the class. The benefit of getters and setters is that they dictate what can be modified and retrieved from the module. So if you use get and set you can prevent overwrites.

Object modules

If you want to use an object instead of a class, you have to take greater care to ensure that the objects are not overwritable. Also you cannot use the # modifier to make properties private.

  • Use getters and setters for updating and retrieving values
  • Use Object.seal to prevent changes to the parent object
  • Instead of using getters and setters, for individual properties you can set writable to be false for properties that you don’t want changed

There are examples of each in the following:

export const age = {
  name: "",
  birthYear: new Number(),
  currentYear() {
    return new Date().getFullYear();
  },
  get age() {
    return this.currentYear() - this.birthYear;
  },
};

Object.seal(age);
Object.defineProperty(age, "aValue", {
  value: 6,
  writable: false,
});
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