cem2ran · April 16, 2020 09:54 · Apr 16, 2020 · Apr 15, 2020 · Apr 15, 2020
diff --git a/reason-0-60.re b/reason-0-60.re
@@ -0,0 +1,2 @@
+// No TypeScript, but statically typed
+let add = (a, b) => a + b;
diff --git a/ts-0-60.ts b/ts-0-60.ts
@@ -4,6 +4,59 @@ function add(a, b) {
 }
 
 // Type function arguments
+//              vvvvvv     vvvvvv
 function add(a: number, b: number) {
   return a + b;
-}
+}
+
+// Type return statement
+//                                  vvvvvv
+function add(a: number, b: number): number {
+  return a + b;
+}
+
+// Accept number or string
+//              vvvvvvvvvvvvvvv     vvvvvvvvvvvvvvv   vvvvvvvvvvvvvvv    
+function add(a: number | string, b: number | string): number | string {
+  return a + b;
+}
+
+// That's a bit silly...
+// a and b should both be numbers or strings, don't mix!
+// Introduce a generic type T that represents number or string
+// 
+// This will effectively result in:
+// (a: number, b: number): number
+// OR
+// (a: string, b: string): string
+//           vvvvvvvvvvvvvvvvvvvvvvvvv  
+function add<T extends number | string>(a: T, b: T): T {
+  return a + b;
+}
+
+// Think of generic types (<T, Foo, Whatever>) as type variables!
+
+// Return an object with the result
+//                                                   vvvvvvvvvvvvv
+function add<T extends number | string>(a: T, b: T): { result: T } {
+  return {
+    result: a + b
+  };
+}
+
+// Make that type "reusable"
+// Notice how we're passing that generic type into the interface
+interface ResultObject<T> {
+  result: T;
+}
+
+//                                                   vvvvvvvvvvvvvvv
+function add<T extends number | string>(a: T, b: T): ResultObject<T> {
+  // The interface ensures that what we return fits the
+  // ResultObject<T> interface
+  return {
+    result: a + b
+  };
+}
+
+// Now you know enough to be dangerous ☢️
diff --git a/ts-0-60.ts b/ts-0-60.ts
@@ -0,0 +1,9 @@
+// No TypeScript
+function add(a, b) {
+  return a + b;
+}
+
+// Type function arguments
+function add(a: number, b: number) {
+  return a + b;
+}
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		// No TypeScript, but statically typed
		let add = (a, b) => a + b;