Last Sync: 2022-06-24 21:00:04

Data_Structures/Patterns/Module_pattern.md

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+---
+tags:
+  - Data_Structures
+  - patterns
+  - oop
+---
+
+# Module pattern 
+
+> Come back and compare this with learning from Node.js
+
+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.
+
+```js
+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;
+  }
+}
+```
+
+```js
+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:
+
+```jsx
+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,
+});
+```