Add TS decorators

src/SUMMARY.md

@@ -18,6 +18,7 @@
     - [arrays](./dev/ruby/arrays.md)
   - [TypeScript](./dev/ts/main.md)
     - [interfaces](./dev/ts/interfaces.md)
+    - [decorators](./dev/ts/decorators.md)
   - [JavaScript](./dev/js/main.md)
     - [ES6](./dev/js/es6.md)
   - [Web](./dev/web/main.md)

src/dev/ts/decorators.md

@@ -0,0 +1,93 @@
+# Decorators in TypeScript
+
+## Overview
+
+Decorators are an experimental feature of TypeScript (and also a JavaScript
+stage 2 feature, meaning they will be soon included in standard JS) allowing
+you to inject specific behaviors to classes, properties, methods, accessors
+or parameters.
+
+This features allow some kind of meta-programming and dependency injection,
+called at runtime.
+
+This is mainly used in libs to add specific behaviors to your own code.
+
+For example, [TypeORM](https://typeorm.io/), an ORM lib, use this feature to give a nice way for
+users to annotate their models, the dedicated char to use decorator is `@` :
+
+```typescript
+@Entity()
+export class Person {
+
+  @PrimaryGeneratedColumn()
+  id: number;
+
+  @Column()
+  lastname: string;
+
+}
+```
+
+## Example
+
+This features needs to be explicitly set as enabled in your `tsconfig.json` :
+
+```json
+    "experimentalDecorators": true
+```
+
+Decorators are just functions, for example, here is a property decorator
+
+```typescript
+class Decorator {
+
+    // Call to the emoji decorator with a string passed as argument
+    @emoji("🦍")
+    name: string = "";
+
+    constructor(name: string) {
+        this.name = name;
+    }
+
+}
+
+// The actual decorator code, this is, in fact a decorator factory
+// It's a high order function returning the actual decorator
+// It's a common and nice way to have access to a larger scope to
+// play with the args passed as params (emojo, here)
+function emoji(emojo: string) {
+    // Return the actual decorator
+    return function (target: any, key: string) {
+
+        // get the actual value
+        let val = target[key];
+
+        // customize getter
+        const getter = () => {
+            return val;
+        }
+
+        // and setter, to add some nice emojos
+        const setter = (next: string) => {
+            val = ${emojo} ${next} ${emojo};
+        }
+
+        // Apply thoose changes to the actual object property
+        Object.defineProperty(target, key, {
+            get: getter,
+            set: setter,
+            enumerable: true,
+            configurable: true
+        });
+    }
+}
+
+const example = new Decorator("hello");
+
+console.log(example.name);
+```
+
+Even if this example is quite useless, it gives an overview of the
+possibilities of this feature.
+
+More ressources can be found in the [Handbook](https://www.typescriptlang.org/docs/handbook/decorators.html)