Type Guards

1// typeof type guard
2function padLeft(value: string | number, padding: string | number): string {
3  if (typeof padding === "number") {
4    return " ".repeat(padding) + value;
5  }
6  return padding + value;
7}
8 
9console.log(padLeft("Hello", 4));     // "    Hello"
10console.log(padLeft("Hello", ">>> ")); // ">>> Hello"
11 
12// instanceof type guard
13class Dog {
14  bark(): void {
15    console.log("Woof!");
16  }
17}
18 
19class Cat {
20  meow(): void {
21    console.log("Meow!");
22  }
23}
24 
25function makeSound(animal: Dog | Cat): void {
26  if (animal instanceof Dog) {
27    animal.bark();
28  } else {
29    animal.meow();
30  }
31}
32 
33// Custom type guard with "is" keyword
34interface Fish {
35  swim: () => void;
36}
37 
38interface Bird {
39  fly: () => void;
40}
41 
42function isFish(pet: Fish | Bird): pet is Fish {
43  return (pet as Fish).swim !== undefined;
44}
45 
46function move(pet: Fish | Bird): void {
47  if (isFish(pet)) {
48    pet.swim();
49  } else {
50    pet.fly();
51  }
52}
53 
54// in operator type guard
55interface Admin {
56  name: string;
57  privileges: string[];
58}
59 
60interface Employee {
61  name: string;
62  startDate: Date;
63}
64 
65function printInfo(person: Admin | Employee): void {
66  console.log(`Name: ${person.name}`);
67  if ("privileges" in person) {
68    console.log(`Privileges: ${person.privileges.join(", ")}`);
69  }
70  if ("startDate" in person) {
71    console.log(`Start Date: ${person.startDate.toDateString()}`);
72  }
73}
74 
75const admin: Admin = { name: "Alice", privileges: ["create", "delete"] };
76printInfo(admin);