Learn React with TypeScript 3

Using a user-defined type guard

In situations where we can't use the other type guards, we can create our own. We can do this by creating a function with the return type as type predicate. We actually used a user-defined type guard earlier in the book when we went through the unknown type.

Let's implement the example from the last two sections using our own type guard function:

1. We have the same interfaces and union type:

interface IPerson {  id: number;  firstName: string;  surname: string;}interface ICompany {  id: number;  name: string;}type PersonOrCompany = IPerson | ICompany;

2. So, let's implement the type guard function that returns whether the object is of type IPerson:

function isPerson(personOrCompany: PersonOrCompany): personOrCompany ...