From 7aa8a034c77ba61688fc2a74a63634ccc7485d72 Mon Sep 17 00:00:00 2001
From: tactonbishop <tactonbishop@gmail.com>
Date: Tue, 20 Sep 2022 14:30:05 +0100
Subject: [PATCH] Last Sync: 2022-09-20 14:30:05

---
 Programming_Languages/TypeScript/Classes.md | 88 +++++++++++++++++++++
 1 file changed, 88 insertions(+)

diff --git a/Programming_Languages/TypeScript/Classes.md b/Programming_Languages/TypeScript/Classes.md
index b332d26..c23c4d7 100644
--- a/Programming_Languages/TypeScript/Classes.md
+++ b/Programming_Languages/TypeScript/Classes.md
@@ -120,4 +120,92 @@ class Programmer implements Person {
 
 ## Inheritance
 
+We can extend classes in TypeScript by using sub-classes or abstract classes.
+
+### Sub-classes
+
+In the case of sub-classes, we use the phrase `[child_class] extends [parent_class]` in the class declaration to designate the inheritance relationship between the base class and the new class that is derived from it.
+
+> A derived class has all the properies and methods of its base class but can also define additional members.
+
+When you instantiate a child class from a parent class, if the parent class has constructor values, you must initialise these in the child. You do this by calling the parent constructor via the `super()` syntax. For example:
+
+```ts
+class Employee extends Person {
+  constructor(firstName: string, lastName: string, private jobTitle: string) {
+    // call the constructor of the Person class:
+    super(firstName, lastName);
+  }
+}
+```
+
+To override or alter a method that exists on the parent in the child, you can use the syntax `super.[methodName]()`.
+
+### Abstract classes
+
+Classes marked `abstract` are similar to parent in the case of sub-classes. The difference is that they are like templates. Several of their methods and properties may be directly inherited by classes that derive from them (just like sub-classes) but they can include 'blank' methods and properties that are placeholders for methods and properties that are defined in the derivation class.
+
+I have found this useful for cases where you want to inherit methods from a parent class but implement a specific method differently in each derivation.
+
+```ts
+export abstract class IndexHyperlinksProvider
+   implements vscode.TreeDataProvider<TreeItem>
+{
+   public activeFile: string | undefined
+   private outlinks
+   private fileSystemUtils: FileSystemUtils
+
+   constructor(
+      activeFile: string | undefined,
+      workspaceRoot: string | undefined
+   ) {
+      this.setActiveFile(activeFile)
+      this.outlinks = this.generateLinks()
+      this.fileSystemUtils = new FileSystemUtils(workspaceRoot)
+   }
+
+   abstract generateLinks(): Promise<TreeItem[] | undefined>
+
+   public setActiveFile(activeFile: string | undefined) {
+      this.activeFile = activeFile
+   }
+  ...
+```
+
+Above we have a base class with a single abstract method `generateLinks()`. The class below extends this base. Note that it passes the parent constructor values to `super` and defines an actual method for the designated `generateLinks()` template:
+
+```ts
+export class IndexOutlinksProvider extends IndexHyperlinksProvider {
+  public workspaceFiles: string[];
+  public context: vscode.ExtensionContext;
+  constructor(
+    activeFile: string | undefined,
+    workspaceRoot: string | undefined,
+    workspaceFiles: string[],
+    context: vscode.ExtensionContext
+  ) {
+    super(activeFile, workspaceRoot);
+    this.workspaceFiles = workspaceFiles;
+    this.context = context;
+  }
+
+  public async generateLinks(): Promise<TreeItem[] | undefined> {
+    const indexer = new IndexHyperlinks(this.context, this.workspaceFiles);
+    if (typeof this.activeFile === "string") {
+      const outlinks = await indexer.indexOutlinks(this.activeFile);
+      if (outlinks !== undefined) {
+        return this.transformLinksToTreeItem(outlinks);
+      }
+    }
+    return;
+  }
+}
+```
+
+> Importantly, you cannot instantiate abstract classes. You can only `extend` them and then instantiate their derivation. This is another important difference from sub-classes.
+
+As with sub-classes, you must initialise the properties of the parent constructor with `super`.
+
 ### `implements` vs `extends`
+
+You shouldn't confuse `implements` with `extends`. `implements` just checks the class as an interface in accordance with the principles of [duck typing](/Programming_Languages/TypeScript/Custom_types.md#duck-typing): i.e the implementing class should have the same properties and methods. It doesn't affect anything internal to the methods or properties. So e.g, if you typed a method parameter as `string` in the base class, this would still default to `any` in the derived class.