User-Defined TypeGuard

Author: Akash52

typescript-masterclass/README.md

@@ -332,3 +332,46 @@ const playlistName = getItemName(
 
 console.log("Playlist Name :", playlistName);
 ```
+
+### User-Defined TypeGuard
+
+```ts
+class Foo {
+  bar() {}
+}
+
+const bar = new Foo();
+
+console.log(bar);
+
+// console.log(bar instanceof Foo);
+// console.log(Object.getPrototypeOf(bar) === Foo.prototype);
+
+class Song {
+  constructor(public title: string, public duration: number) {}
+}
+
+class Playlist {
+  constructor(public name: string, public songs: Song[]) {}
+}
+
+function isSong(item: any): item is Song {
+  return item instanceof Song;
+}
+
+function getItemName(item: Song | Playlist) {
+  if (isSong(item)) {
+    return item.title;
+  }
+  return item.name;
+}
+
+const songName = getItemName(new Song("wonderful wonderful", 300000));
+console.log("Song Name :", songName);
+
+const playlistName = getItemName(
+  new Playlist("The Best Songs", [new Song("The Man", 30000)])
+);
+
+console.log("Playlist Name :", playlistName);
+```

typescript-masterclass/src/app.ts

@@ -17,8 +17,12 @@ class Playlist {
   constructor(public name: string, public songs: Song[]) {}
 }
 
+function isSong(item: any): item is Song {
+  return item instanceof Song;
+}
+
 function getItemName(item: Song | Playlist) {
-  if (item instanceof Song) {
+  if (isSong(item)) {
     return item.title;
   }
   return item.name;